His trainer won't let him workout due to a knee injury? He needs a new trainer for starters if at all. A busted knee won't stop anyone from doing:
6)Hanging Leg Raises
10)One legged squats, pistol and/or airborne style with uninjured leg
Thats ten off the top of my head hitting the entire body.
Nutrition is another major component that is still 100% under his control. No reason to sit around and pig out.
Rehab should be about maximizing areas that aren't injured instead of doing nothing until the injury heals.
Best to him regardless on a full recovery...