Look, professional sports photographers don't use Nikon's 5200 nor the Canon T5i. More and more, they are using the Nikon D4. The fundamental question about brand choices is a highly personal one. I've been and still am a Nikon shooter. I started before Nikon HAD slr cameras with Nikon's superlative S-2 and SP models (still have them and still shoot them on occasion). Canon makes some excellent gear. But it seems to me that you choose canon if you want a video camera that takes stills and you choose Nikon if you want a still camera that shoots video. Most of the old guard are still shooting from the dark side.

Want even more options? Consider the slightly older 80-200mm f/2.8 Nikon lens. No VR, but we all photographed for years with out it and got good results. The 80-200 is slightly heavier but loads less expensive and don't be shy about used gear, particularly Nikon and particularly lenses. Glass doesn't wear out and abuse can be spotted. And, you should be able to find a good 80-200 for less than $800.

I would suggest that she might be best served by learning to use what she has. There are no flies on a D300 and if she is not satisfied with her results, I would suggest a need for more learning rather than a better camera.
I've been a Nikon shooter for more than 50 years and a pro for most of that time. I am also a member of NPS and in all that time, I have had to send one body for repair after I dropped it on rocks. Pro level Nikons are tough as they come and the 300 is a great camera as is the 7100.
I'd say master what she has. The pixel density in a cropped sensor camera gives her a wildlife shooting advantage over full frame.

Leave the guy alone! He's apparently not doing anything illegal if the subjects are over 21 and it sounds as though they are. I'd wager most posters on here don't get asked to shoot much at all for pay and most are hobbyist.
Now...are you really shooting porn or boudoir? Porn involves a sexual act or images that are more gynecological in nature. Boudoir is just sexy, maybe with some pink parts visible. I suspect your new stripper clientel is asking for promotional images, which I would class as boudoir.
Be VERY cautious about age and ALWAYS have your assistant present. Be absolutely certain to get a release on both the model and, if appropriate, the location as well. Then file copyright within 3 months of the shoot.
Otherwise, enjoy!

Well, I Am 70, so congratulations on making it to this advanced age.
No one seems to ask what you like to shoot and what do you find missing in your photography that you hope to achieve with a new body (the camera, not you. That's a whole 'nother subject!). If you are seeking better low light performance, get the FX body. You might also consider a used 700 which is the way I would go having shot with them for years before I upgraded to D4's and 800's.
On the other hand, if you have much in the way of glass or if low light is not your holy grail, then stay DX.
The camera you have now can best be described as a Ferrari engine in a Pinto body. The sensor in that camera is pro class, so I would not consider a 5100 as I don't think you gain enough to justify the expense.
I've been a Nikon shooter since 1957.

I took the liberty of touching a few things...contrast, exposure, white balance and the clone stamp tool. Basically, a good shot. Oh, yeah...took the bags out from under her eyes and lightened them a bit.

One more...white balance needs to be fixed and the image is slightly (1/3 stop) underexposed.

Couple of things...
By using the perspective control in Lightroom 5, I was able to make the brickwork appear to be level. However, comparing the seams in the brickwork above and below the mantle shows that the brickwork above and below are NOT parallel!
Indeed the short term fix to the problem is to be sure that the camera is in alignment with whatever would reveal a slight cant to the image and then to shoot in a manner that gives you the option to adjust the level and not crop too tight on the image.
I would also like to see the little girl's head a bit higher so that not so much of her is hiding behind her arm.

Hell, I'm not the one who got it wrong! Canon has been misspelling their own name for years. And the way Nikon got a foothold in this country is Life photographers were sending back incredibly sharp images fromKorea. they had parked their Leica's and picked up Nikons unbeknown to Life staff and were using them. Word spread!

Look, it's the skill of the photographer that matters. Cannon makes some excellent products and so does Nikon. The D7100 test out at the 17th best sensor according to DXO Marks. First Cannon is 24th. For lenses, tests are done on specific and multiple bodies. First Cannon combo is 56th, behind 16+ Nikon tests. Cannon does video better than Nikon and marketing better. Nikon got behind because they were on top of the film world and reluctant to leave for digital. Cannon got a head start and they make very good gear, but I've been a Nikon shooter since rangefinder days and I have never had a bad camera or lens. Now I shoot multiple D4's and a D800E. At challenging venues, I can produce shots of a quality that cannot be matched for iso, low noise and focusing ability.
But the best advice is to buy what feels good to you. Either one will not restrict your ability to produce remarkable images. It's up to you.

I'm a Nikon shooter and have been since 1957, so I'm a useless source for Canon information, but there are no flies on the Nikon 70-200 f/2.8 shot with a 20E III tc. Softness is around the edges and I shoot D4's which can focus in low light to f/8. There is another trick lots of sports shooters use. Lie to the camera and tell it to shoot cropped sensor. On my D4's, that gives you an additional 50% focal length. So...TC 2X on 70-200 f/2.8 shot as a cropped sensor gives me an effective 600mm f/5.6.
Here's an example...4.7% of original image size, shot with 300mm f/2.8 and 2x TC, all Nikon.

Formula One Ferrari-Austin 2013 GP