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.