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I wouldn't put it that way. He was the one in the relationship and it was his responsibility to treat his GF like a GF and treat you like a friend, but you said he took you to movies and out for dinner - that's usually something couples or really good friends do. If my boyfriend took another girl out, I'd definitely be jealous and I'd hope he had a good reason, like that they were childhood friends or something. While I'd be annoyed with the girl for going out with my guy, I'd be more upset with him for actually taking her out. So you didn't ruin their relationship... I think the girlfriend's anger was misplaced.