Doesn't matter did he flame you or somebody else, if he flamed, he will be banned (if somebody reports). You can aswell report someone in the game that you did not play, if you want. It doesn't matter at all, if rules are broken, he shall be punished.
as efko said its moderator decision... but when i play with fiends i curse everything i can think off. if they dont want to report me why would someone in enemy team(or even someone who isnt ingame) report that?