I’m in a sticky situation. Please help!!

So I met this guy like 3 years ago at a family party we liked each other and had been friends for a year and he asked me to be his girlfriend we hung out a lot. And we talked for a bit it was long distance nothing ever happened. Butttt it turned out he had a gf already of 6 years!! 🙄 so the girl called me and was all mad and I was like girl I had no idea. And we started talking and she’s like you can be with him and I’ll back off. Well I didn’t want to be with him I knew they were still together. Anyway we stopped talking for months and she starts texting me asking for advice with him. And then we stopped talking months pass by and she’s like wanting to be my friend. She ended up getting pregnant with his kid and the kid is a year old. She’s messaging me on Instagram and texting me saying she wants to be friends. We like talk about makeup and life and stuff but I feel weird cause i used to have something with her bf and if my bf right now found out he’d be pissed and wanting to know why I’m talking to this girl and I kinda want to be her friend i have no friends but if we ever hang out us 4. It would be weird. So do I stay friends with her or how do I let her know that it’s weird?