The problem is, there is no public transportation directly to the Bretton Woods area. There is Greyhound/Concord Coach to Littleton, NH which is about 20-30 minutes away, and I think there are taxi services available in Littleton (at least according to Google), so that might be one option to explore.

If they are staying at the Omni Mount Washington, there is a shuttle service available, but I'm not sure if it would take them as far as Littleton. Might be worth asking, though.