Update: I soaked said Jaeger barrel for a couple days with 3-1 penetrating oil, and it cleaned up real nice again, except for the last 2" at the breech, which looks permenantly pitted. He must have left it loaded for an unusually long time. It shouldn't affect accuracy. He shot really well with it last weekend, even before I cleaned it up!
I shall go over his CLEANING AND MAINTENANCE procedures with him and provide him with samples of what I think are good lubricants.