Its not like Phil wants to marry the girl...

She was annoyed with her boyfriend cos he has insecurities but he let them take over..
She calls up Phil.. he takes a moral approach and refuses to fuck her whilst she is in a relationship.

She ends the relationship. She is now a free agent. Phil can do as he pleases.

I fail to see where the ex boyfriend was "in his rights" in any way. Its one thing to be insecure about your girlfriend's ex fucks, another thing to have a conversation with her about how he feels, but if he wouldn't drop it and gets all weird.. well it isn't healthy for anyone