Hey, I did the same thing just two weeks ago... 3 hours of guitar coupled with 1 1/2 hours of octaves at double forte... totally killed my right hand. It left my right hand limp and useless but now it's feeling normal, though it still hasn't regained it's former stamina yet. I can only play for about 10-15 min at a stretch, so I think it will take another week or so before its completely back to normal.
Your friend seems to have had it worse, though, my wrist didn't swell. I really suggest he rest the hand and not touch the piano, or indeed, use his right hand to do anything at all. If the gig is not too strenous, I suppose he could try to do it, but in the meantime, don't even use the injured hand! It's really up to him, though, but I'd really suggest resting it.