CONFUSED BOY HELP!!

Okay so I dated this guy for a few months and I really liked him but he was talking to my best friend at the time and she didn't tell me, until after I dumped him, I've been hanging out with him for abit recently but he always still texts her and I feel like he still likes her, NOTE he is a massive ass flirt!!! I don't want to go back to him but it just frustrates me and she falls for it,