I would chose a Shimano Deore rear derailleur, they are 9spd, but 8spd works just fine with it, lots of people do that. I plan on upgrading my Sirrus to a Deore rear derailleur someday myself, and its 8 speed. Or you could get a shimano Alivio rear derailleur, Should work fine unless your friend plans on offroading: http://www.jensonusa.com/store/produ...erailleur.aspx
Or, if your friend likes gripshift, here are more SRAM made, shimano compatible shifters, http://www.jensonusa.com/store/produ...p+Shifter.aspx
you would have to buy an 8 speed rear and index or "microfriction" front. I would go with the microfriction front if gripshift are ok, it lets you trim your front derailleur so you just about never get chain rub!
I link to Jensonusa becaus I deal with them a LOT, and seem to always get decent prices there. E-bay is another choice, but especially shifters, why ebay if you can buy brand new gripshifters for $20 or less from a reputable dealer? I would try to find a DEORE rear derailleur on ebay though, Cant seem to find many online right now, from the dealers I check at least.
Good luck, and you sound like one heck of a friend