I can appreciate that but doesn't your friend feel exploited? Doesn't he feel weird being reminded that people with his condition are seen as fetish targets? Or that they can be added to a game in order to drum up hype that otherwise wouldn't be there if the characters were "normal"?
Eh, this is sort of like saying that a character who, say, lost their parents in a car accident when they were young and the story centers around him dealing with that is "exploitative." Either way you have a character who has an exceptional abnormal circumstance in their life that was beyond their control and which requires special attention to deal with. Drama is all about exploring situations that you might not deal with or even be familiar with.
For me, I feel like just trying to pretend that a disability doesn't exist is also quite damaging, but all in all it's a sensitive subject for all involved and some people will have reservations when these types of subjects are dealt with. Totally understandable, so I don't really think you're crazy for feeling this way. Nevertheless, it's not your place to force that opinion on others. If you don't like it, don't play it, make your position known if you feel you must, and then just let everyone else decide what they want to do. Nothing more is needed.