my boyfriend is not over his ex

my boyfriend had told me that a girl had sex with his friend while he was locked up but I never knew the name of the girl...I caught him flirting with a female & it turns out that it’s her. he tells me that he wants revenge on her? wtf does this mean please someone tell me😂 is this an excuse or what? I tried to tell him that the girl doesn’t care about him n he gets defensive like lol wtf. He also hung out with her twice since he’s been out.