Odd the 7D should have had good enough AF if paired with a good 70-200mm; though for indoor action in very low light chances are the 5D III would have been superior (since larger sensors have improved low light performance and since the 5D III gained a similar AF system).
That said if low light work is a major part of what he's doing then Nikon currently has the significant edge with their ISO sensor technology that they got off Sony.
I'm quite surprised to hear about your boyfriend's preference for the 50d over the 7d. I use both, having bought the 50d as a second lens to lug around, and in my experience there is nothing that the 50d does better than the 7d. In particular, having a much better AF system in the 7d (with the ability to change AF zones, and changing focus speeds to better match specific lenses) makes it in my eyes a godsend for shooting fast moving subjects.
I know absolutely nothing about photography so I’m not entirely sure what was causing the problem. Luckily enough he should be working with better conditions now, primarily outdoors, when photographing animals. Although I’ve heard other people say that the 7D really wasn’t too great a camera (a staff member we spoke to in a camera shop the other week wasn’t very surprised that it hadn’t worked out for the purposes he had needed it for) .