If the shifter really is dead, you can still make use of the front derailler as a chain guide. It sounds like whatever gears your friend is using is causing the chain to be at such an angle that the chain rubs on the front derailler cage. You should be able to adjust the cable tension to position the derailler cage in a spot where the rub is minimized giving him use of at least some of the rear cogs.

With that said, MTB style pod shifters are shockingly cheap so assuming that's what he has and that the shifter is the problem, you might consider just buying a new one. Sets are available for $20-30 and include new cables and housing.