What exaclty is swollen? the wrist? a finger? or some other part of the anatomy...
as i guess it's the wrist or the hand, he's probably doing someting wrong, very wrong, tensing up unnecessarily and playing pieces when he doesn't know them and therefore not taking precautions to remain relaxed while he's playing.
It's difficult to say anything particular without knowing the person and actually seeing the person of course, but the big guideline would be to completely unlearn and relearn all his technique (easier said than done!)