I've done it, although not with anything as complex as a whole motherboard. I x-ray the PCB, use a multimeter, visual inspection, datasheets for the ICs, and a bit of intuition. It takes a lot of patience and a good eye but it's certainly possible. If you don't care about reusing the board, removing many of the parts makes it easier to see what's going on with the traces. Most of the time there isn't much on the inner layers besides ground and power planes, most of the traces connecting parts are on the surface.
If the guy is willing to part with the schematics, what harm is there in scanning and posting them somewhere? There is no commercial value in schematics for 20-30 year old computers, the only people interested will be hobbyists trying to keep vintage machines going.