I'm with you on this hairball approach. A roof section is going to be insurmountably heavy to remove whole, then re-use after the support walls are built. The existing roof structure would be compromised, would limit the architectural options (overhangs). The replacement roof has a different roof slope and size. No way. I wouldn't even consider it. If your buddy wants to strip the roof and save scraps of lumber and plywood for re-use, so be it. Personally I don't even like working with used materials due to damage to saw blades, inconsistent size, etc.