Worried about my best friend

My best friend has always been super close to me since I met her in 6th grade. About 8 months ago, she started sleeping w this guy as a Friends With Benefits Type situation and has been dating him for 3. Ever since the beginning of it, I thought it was just weird bc the guy was friends w a guy who she thought had a crush on her but had a secret relationship W HER MOM🤦🏼‍♀️

Anyways, she started to tell me he would act strange; really possessive, sometimes affectionate, but would always switch between caring and clarifying they were both single so he didn’t care. And when she would ask me, I would tell her that it was too much drama.

Then one day, after normal convo, she randomly stopped talking to me. I tried to reach out to her for MONTHS, since October, and just 2 DAYS ago, she gave me a different contact to message her. She says they “deleted social media for each other and rarely get on it”. But she’s been opening my messages?? I tried to still act normal and her boyfriend came up. I told her that I was happy for her but told her I still think she should be careful bc taking 4 months to admit feelings when you already are sleeping w someone just didn’t settle right, and even my boyfriend agreed.

But get this: she says “my opinion didn’t matter to her bc I was being negative”. 😳 Wtf! I’m so hurt someone so close would chose a guy over me; one that played w her emotions so many times at that!

Any advice?