The High & Mighty hangers look interesting, I think I’ll order a set just to try out. They may not work, though, or may require hammering, as whatever it is that his walls are lined with in retrospect I don’t think is drywall – it’s really hard stuff. I think the best solution would have been to use pairs of ordinary hangers (the kind with the nail going in at a 45° angle) but with the fancy Ook Pro nails, which would allow them to be hammered in completely. As it is, I’m going to suggest that he reinforce the heavier pictures with an extra hanger of this type.
Thanks to everyone for all the feedback!