Yes, this is not just resetting the stays. You'll have to ensure alignment, add a connection for a derailleur hanger. Given the frame geometry, the dropout is unlikely to be standard (but perhaps someone makes them?). Also: Brompton claims on their site that the frames are brazed. If so, there's a reason. Perhaps they're made of chrome-manganese steel (like 531) and welding is problematic. So make sure that your guy uses the right technique to make the joint.