This is an idea that if certain planets are aligned, the moon is full, and the stars are smiling, could turn out ok. However, my experience is that when an inexperienced photographer does something like this it often does not end well.

Things that can go wrong on this most important of days for a couple:

1. Gear failure. It happens. It sounds like he/she does not have backup gear, just in case.
2. Not having a contract - even if he is doing it for free, there needs to be a contract that describes a scope of work - what shots he/she is expected to get, and when the shooter is expected to deliver the proofs
3. Inexperience with gear - nothing worse than waiting for a photographer to stop fumbling around with the camera.
4. Dropped lens, left the battery in the charger, left the memory card in the card reader - it happens.
5. On the day of the wedding, the Bridezilla emerges - it happens, and unless he is a super-dooper people person, she (or her mother) will be completely unmanageable.
6. Not having a Plan B, Plan C etc - wedding party is late, it starts to rain, the wind kicks up - things you have no control over but happen nonetheless - contingency plans need to be in place "just in case" and need to be discussed beforehand.
7. A single flash and a work light will not do. Points to lack of experience. Shooter needs to be fully familiar and comfortable with low ambient light and bounce flash. Fast lenses are not the answer. You can't shoot a wedding at F1.8 - shallow depth of field will spoil a lot of the shots. Good shooting technique, even better post processing skills so that high ISO shots are not too noisy. I doubt the minister will allow you to set up a work light in the church. Many won't even let you use flash - too distracting.
8. MWACs and GWACs - moms with cameras and guys with cameras - nothing worse than having a great shot spoiled by someone's cellphone and hand right in the middle of the scene. This needs to be discussed and if the bride and groom do not agree to an "unplugged" wedding, where there will be a designated time and place to take shots of and with the wedding party and the couple, they will need to understand that your friend is not responsible for the results.
9. Not having enough batteries, memory cards.
10. Not having an assistant - I would never personally take on an assignment like this without an extra hand. Just having an extra hand to hold a pole mounted flash held by an assistant will greatly improve the results.
11. Permits - if some of the shooting will be done outside, at a park or somewhere else, a photographer will need to secure a permit.
12. Insurance - what happens if something goes wrong - you are on someone else's property - you are liable if someone is injured as a result of your negligence. Insurance is a must.
13. What happens if your friend goes out to dinner the night before the wedding, eats something that causes them to get sick and wakes up the next morning with the sluices open at each end? This speaks to contingency plans in #6.

These are some of the gotchas - all of which I have either seen happen or have happened to me.

I suggest your friend not take this on. This is too important a day. Right now, they are thinking of the money they'll save. After the wedding they may end up being really disgusted at the results, and there goes another personal relationship down the tubes. A wedding shooter has to be confident, experienced, and extremely well-organized to pull it off. And he/she doesn't get a do-over if things don't work out.

One last thing - has the couple contacted the caterer, the florist, the tailor, the reception hall, DJ/Musicians, etc to see if they will get a discount? Probably not. The couple is not placing a proper value on photographic services, and this is a red flag. They are expecting great results for free.

I cannot say it enough - your friend should not accept this. Too big a responsibility. I there is time, your friend should at the very least find a wedding photographer that is willing to take him/her on as an unpaid assistant, just so he/she can get a better sense of what is involved.

The kit lens is not very sharp. I dont have any experience with the telephoto but my guess is that a fast prime would be better.

He should look into renting the right lenses and some external flash units. But if he does not already own them he probably does not how to use them in a wedding context.

If there is time, he should at least try to observe a wedding done by a pro so he can get an idea of what is involved and how its done.

I wish everyone involved nothing but luck, but in wedding photography, like just about anything else, you get what you pay for.

Is this meant as a joke? I mean even I a seasoned wedding photographer know not to tackle a wedding without being prepared. Although after seeing the wedding photos on display at a local photographers place of business I think that one professional photographer needs to go back and learn more about photography. Your friend could be doing the bride and groom an injustice especially showing up with a shop light for group pictures. Please advise your friend to be courteous and deny the bride and groom his burdening offer of at the best ok photos. I am sorry if I sound harsh but I have been there with a bride asking me to help her create an album with what other shot at her wedding. She would say as I say do not let him impose on the bride and groom what he knows nothing about. I have shot over 1500 weddings before digital showed up. The equipment you need will be at least 2 camera bodies with a spare battery. As for lenses he can do ok with the two he has if he knows how to use them. Flash would be best rather than shop lights because unless you know the color balance of the lights you will either get a very yellow shot or a very green one which depends on the light source. Flash is considered daylight balanced. You need two flash units large enough to cover groups of more that three people. Plus you have to have done a few test shots and figure out how the flash guns talk to the camera and to each other. I purchase 2 not long ago but I have yet to figure how to make them talk to each other. (I should have gone Nikon) I did not retire from wedding photography I was forced out by digital and now I am shooting digital and I am a few strobes away from a comeback. I took many years and photo courses to achieve proper posing and lighting. I worked with the best until I mastered the art. So I must say tell your friend he may make an enemy out of the bride and groom if what he gives her is shoddy work. I have shot the bridezillas and they come back to haunt you when all is done even if your work is far above the best. Many amateurs know this and will refuse to shoot a wedding so I advise your friend to leave well enough alone and not play like he knows how to shoot a wedding.

A friend who owns a Nikon D7100, and owns only two lenses, a Nikon 18-55mm kit lens, and a FX 28-300mm, wants to save his sister money, from not paying a pro photographer to do a small Wedding. He is an amateur just like me. I asked him, why didn't the Bride and Groom's family chip in to hire a pro photographer. He said they were already paying for other preparations to the Wedding. I told him those 2 lenses he owned were not going to cut the mustard. He does have an external flash unit. I offered to loan him my prime lenses of 35mm f1.8 and 50mm f1.8. And an outdoor plug in work light he could use inside the small Church. He has never done a wedding before. He said he will have two young laborers to help out. He says he can handle it. I told him to prepare for a long day when he does it. Reception included. I'm glad I'm not doing it.

A friend who owns a Nikon D7100, and owns only two lenses, a Nikon 18-55mm kit lens, and a FX 28-300mm, wants to save his sister money, from not paying a pro photographer to do a small Wedding. He is an amateur just like me. I asked him, why didn't the Bride and Groom's family chip in to hire a pro photographer. He said they were already paying for other preparations to the Wedding. I told him those 2 lenses he owned were not going to cut the mustard. He does have an external flash unit. I offered to loan him my prime lenses of 35mm f1.8 and 50mm f1.8. And an outdoor plug in work light he could use inside the small Church. He has never done a wedding before. He said he will have two young laborers to help out. He says he can handle it. I told him to prepare for a long day when he does it. Reception included. I'm glad I'm not doing it.