I need some advice.

So I've been together with my boyfriend for a couple months and we've dated before in the past but just recently got back together. While were broken up my friend started talking to him after I asked her not to and fell in love with him but they never dated. That was about 2 years ago. So recently my bf and I have been hanging with my friend and they've been talking on messager(we were in a group chat but she switched them to a private chat) and I didn't think nothing of it. So I asked her if she was talking to him and she said no but he said he was. I asked her again a week later as I knew for a fact they were talking and she said no haven't spoken to him since group chat. So I'm pissed off cause she's lying to me but i dont want to go off so i just shake it off. But when i called her out she says that he just talks about me but that's a lie because he said they were making plans for all of us to go camping which I had no idea about apparantly it was her idea and she was suppose to talk to me about it but she never even mentioned it to me! And my best friend thinks something fishy is going on but I ignored it. My bf being truthfully but my friend is lying to me. Everytime I ask if they're talking he says yes but she says no.