Hello,
I too have had to modify my piano playing (and life) because of MS. I used to play at graduations, church, etc., but MS has affected my quick repetitive motion and the ability to change positions quickly. Now I just play simple arrangements that don't require either of those motions (ie. no Chopin). Actually, I kinda enjoy having an excuse not to play in public. Seriously though, I'm lucky in that my livihood doesn't depend on being able to play the piano. As far as enjoying music, I've really gotten into MIDI files and listening to new kinds of music on the Internet.
You didn't say which hand of your friend's is affected. If it's the left hand, then a modified bass is a very workable option. If it's the right hand, he would be limited to one finger melodies in jazz-type playing. I learned while teaching children to play piano that it seems to be a marked skill level when they are able to "change hand positions" or in other words, move to another place on the keyboard quickly. There are many arrangements that really don't require this. Playing simple arrangements by ear sound good when you "spread out" or try to use a wide range on the keyboard. The melody line can be a single note up high, and the bass can be a very low root note and a mid range chord. (this can even be a two-note chord.)
I would really suggest that he use this time to learn different types of music, maybe also differnet instruments. There are some great one-day workshops for blues playing. Also look into various dulcimer and folk workshops. I've learned a lot about arranging by making MIDI files of favorite songs.
Let us hear from you again.
Mary