The problem is your friend and a cheater both know his serial number, which is used to compute the GUID. If they unban one of them, both are unbanned.
It's likely your friend downloaded a program to "optimize" cod4 or something. Also, never ever have someone create a rank file for you because it requires they have your serial number. I'd be that's how most cheaters get new GUIDs and normal players get theirs stolen.