My sympathy to your friend. Gene Ellison, who used to be active in this forum has a site. I haven't visited it in a long time; but it had links to various slide guitar sites. The URL is http://home.att.net/~Ellison-TexasHe is a real expert on the instruments and some of the styles and you can contact him via his site to ask specific questions.

By the way, I think it is possible to play slide guitar with just a slide; but your friend will have to master a number of open tunings and be ready to capo to get the other keys.

If he goes for the harmonica he can look at http://home.stny.lrun.com/gnstudio/basics.html or http://www.volcano.net/~jackmearl

I am far from an expert (in fact I am a rank beginner!!); but I find the diatonic harmonica very satisfying to play.

Murrau