I think 1 and 3 are the best (I would personally pick 3). It wouldn't be a violation of privacy as long as you didn't mention your friend's name or any other identifying information other than the fact that he is wheelchair-bound.
There are also a lot of recent threads about this if you care to look back through the last few pages of this forum.
I like the third one as well, especially if you have mentioned the other two. I wrote mine about my psychology background and my knowledge of and experiences with mental illness (in others, not myself), particularly my drive to eliminate stigma and increase education. I think it can be valuable to show that you have an understanding of a specific patient population, since that's something unique you bring to the table.
Hopefully I'm not taking this the complete wrong way, but I think you could make a good essay out of that.
I vote number three as well. Just make sure to call out specifically what you learned and how it would apply to/benefit your fellow classmates and your medical education. I did something similar with my diversity essay talking about growing up with a dad with Parkinson's Disease and what i've learned about the social implications of disease. Good luck!