I'd second recommendations for slide guitar. Played either lap-style or upright, I don't think the missing digits as you describe them should be too much of a disability. Slide guitar, usually played in open tunings can in my opinion be one of the most beautifully expressive forms of music and is adaptable to many styles.

I don't have a firm position on the left vs. right handed debate, but I would add I'm left handed and have played "right handed" from day one, on guitar, banjo and mandolin.

Good luck to your friend!

Pete.