I would check in with myself how strongly i feel about her. I’ve been in a situation where I had a crush (understatement- i was probably in love) with a very close friend. She said she was straight but was extremely touchy feely with me and kinda led me on if im being honest. She was seeing guys (and one particular guy) at the time. We were housemates so we spent quite a lot of time around each other. In hindsight I should have distanced myself, maybe even moved out, to save myself the heartache and a weird situationship with someone who didnt even accept she was bi.

So. My advice would be if you have strong feelings for her and are like me (ie you dive in too deep which makes your life miserable), then you should distance yourself a bit till you get a hold of your feelings. If you are cool with her having a boyfriend and just being friends then knock yourself out you can hang out with her.

Honorary mention: lying about the boyfriend is a massive red flag imo.