He won’t stop talking to a girl I don’t like

My bf of 5 years won’t stop speaking to a girl I don’t like. Firstly, she did not know he had a gf in the start so she didn’t know her boundaries with him, secondly, he told me that she likes him but he doesn’t feel the same way about her. And thirdly, when I communicate my feelings to him about it, he says “it’s nothing to worry about” and that he doesn’t feel anything towards her. I caught him several times speaking to her and he’s sometimes shady with his phone and won’t let me introduce myself to her (to be friends with her).

I have her number and I did call her and speak to her just for a peace of mind.. and she said that “I should take it up with him if it’s a problem” how bitchy of her to say!!!!!!!! He threatens me when I tell him I’m gonna call her up again and ask about whatever they had going on. Lately he said he’s stopped talking to her to work on us, but I’m scared he’ll run back to her again. He would always talks mad 💩 about her and say nasty things about her but would still speak to her??

Any advice?