Posting a game-link will help people figure out what happened. If his turns take forever, it's not him being booted and turning into an AI; the AI commits in a matter of seconds. He might have stalled and let the autopilot play though. For that, he needs to be a member, so check that. A player who's been taken over by an AI (if allowed by the game settings) will not surrender until it is eliminated.
considering what he said he is probably a member and used "Wunderwaffe" wich is an AI. however Wunderwaffe shouldnt be able to get a comeback. Its terrible when it has income disadvantage and doesnt know what to focus.
if he isnt a member he probably has some friend wich told him what to do thats omewhat decent at the game.
