Wow. That's a pretty serious looking progression. Did he lose his thumb? I killed a brown recluse last summer, I think I will re-check the house.
Don't know. I found the link via Google. ;) Those aren't the worst I've seen however.
The skin is pretty thin on the back of the thumb, and as long as the tendon is OK, a graft might repair the area. Sure going to leave a mark though...