Yah, because swimming and lifting are intense, and increase blood flow A LOT. The pressure and strain can cause damage to the brain when its trying to heal from a concussion. *from what I understand from what my doctor told me
I was doing some light biking and incline treadmill walking, low intensity but still gets that blood moving. But I also lost a lot of weight when I came off, I dropped from 225 pounds to around 175 and it wasn't fat weight either. I just got skinny, but it may have been effected by the post concussion depression I seemed to have for several months afterwards as well.
Good luck to your buddy man! Just be smart (unlike I was at the time) and don't stop eating, just clean up that diet if he still can't lift and workout intensely.
Glad to hear he's doing better though.