Help guys! I've been tracked!

Help!!! Today I've been tricked! I played Call of Duty Modern Warfare 2 Multiplayer, and I entered a friend's game. I'm level 51 prestige 0. Suddenly a guy, Steam account name xBen117 (Steam Community :: ID :: xBen117), asked me want a private match or not. I said no problem, then he added me as a friend, and invite me to the game. I entered, and he said I can make you prestige 10 and level 70. I said I don't need it, because it's not a regular way to get to the high level. He then said it's just mod, it can bypass VAC, I also said I don't want it, I just want to play the game, I don't want any shortcut or mod, cuz I can get banned. But then he said I've entered a modded lobby, and I've been deranked. I said I didn't want it, why you do that to me!? Then I left the game, and I noticed my level has changed to 70 prestige 0! I don't know what he done to me, I didn't know what did he use, and I don't want it, but then I noticed he unfriended me! And he still continue to play the game! His in-game name is Trollface.jpg when I entered his lobby, but not xBen117, I don't know why! I've contacted Steam and VAC, so what should I do now? I didn't cheat and hack! I just don't know why he can mod my level in the game! What should I do now? Help!