Oooh how does this sound?
Dear william,
you dont know me at all, aside that im friends with your boyfriend. Perhaps i should tell you this though. During the course of his relationship with you, he cheated for a period of two months. With me. Just felt i should let you know that he told me i was single. I have no interest in the cheating jerk but to see him suffer. I suggest you confront him, cause he has a nasty habit of trying to do everything that moves.
Sincerely, kyle.
Hows that sound?