This should work fine. A lot of people who think that the gearing on road compact cranks (typically 34/50) is too high for really hilly terrain (or long distance/endurance riding) look for a way to have a super-compact crank (something like a 30/46) and use the option that you describe - only using the inner and middle positions of a triple crankset. There are other options, but this is normally the cheapest and simplest.

With a 30/42 combination, most front derailleurs (double or triple) should work OK, although some will work better than others. Also, try mounting the derailleur at several heights until you find the one that achieves the best results (the best-performing position is occasionally with the outer cage significantly above the largest ring, but it's hard to predict).

If the person wants even lower gearing, then consider switching the inner ring for a 26 or 28 tooth ring. I have a commuter bike with 26/42 chainrings on a road triple that works very well. Since it is for commuting, I've added a chainguard in the empty outer ring position to keep my pants away from the rings and chain.