My gosh! There was a bowler like that at my house 2 years ago. Knocks the heck out of the pins. I can't imagine laughing at anyone who is physically or mentally less well-equiped than me.
However, maybe your friend could develop a little comedy routine that he does with each opposing team prior to the game, to help them get adjusted to his differentness. I say comedy because nobody wants to hear about his medical woes, but everyone likes a person who doesn't take him/herself seriously.
Something like this - He throws a practice shot, turns around and says "Hey! Anybody got a spare knee? Mine doesn't seem to work." He throws his second practice shot and turns around and says "Wow! That baby was airborne!" Third shot, he turns around and says "I think I'm bending too much at the knees."
This breaks the ice, and then when folks on the other team ask you - or if they don't, you can offer - just tell them he's recovering from surgery and can't bend the knees. This all sets up for the game, at which time they should not be laughing at your friend but cheering him on!
A little personality goes a long way.
"If it ain't workin', you're either throwing the ball wrong or throwing the wrong ball."
"Follow the oil!"
"Dry lanes ain't worth a shot!"
"I love the smell of lane conditioner in the morning!"
current avatar is Gabby Hayes. Looks a lot like me!