There are a lot of really good previous examples of gamers needing alternate configurations that have been popular on the Internet over the years. From gamers that use their head for controls[1], to folks in the same position of having the use of only one hand[2]. Whether it be customized controllers, or just having to re-map a lot of keys or buttons, there doesn't seem to be a barrier to enjoy gaming.

You should let your friend know that they are not alone and they almost certainly can still have plenty of enjoyable gaming ahead of them.

[1] - http://www.nbcnews.com/tech/video-games/disabled-gamer-shows...

[2] - http://www.reddit.com/r/leagueoflegends/comments/120per/ama_...