my boyfriends best friend..

yesterday my boyfriend and I fighted because of his girl best friend.. he went to a party with her and they fighted and now he canceled her. I wanted to know why but he didn’t tell me till yesterday. They fighted because she asked him if he wants to be sex buddies and that she has feelings for him. They know each other for 5 years now and he told me that he was in love with her at some point. So when he found out that she loves him back at the party he got mad about it. I didn’t really understand why, why does he still care about that?? Why does this still breaks his heart and why would he get mad? He was in love with her 3 years ago?? Should I be worried ?