<human>: B [22/f] and I have been friends since high school. Obviously drifted apart a bit during university but always stayed in touch and started getting really close again once I moved back from school. 

The more we would hang out, the more often we'd end up hanging out with her boyfriend, let's call him SS, as usually happens when a friend has S/O, it's usually not a big deal. The problem is that he started being rather flirty with me, complimenting my ass via text after we'd hang out and eventually "drunkenly" propositioning me. 

I told him over and over that he was lucky to be with B and that I didn't appreciate his behavior. So he let up a bit and I thought it was all good. Then we went on a double date and he was SO awkward - even my bf picked up on it. SS actually ditched us early. I texted him about it later and he gave some vague answer like "I just wasn't having a good time". This was 6 months ago. 

Every time I tried to hang out with her, she'd want to have him along to the point where if I stressed that I just wanted a girl's night, she'd get annoyed. They live together so even a girl's night in would involve him being around. So I just stopped trying to see her or anything. 

She messaged me this weekend to wish me a happy birthday but also mentioned that she didn't understand what happened to our friendship and how it was upsetting but she couldn't keep trying. I'm so upset because she is an amazing person but I feel like there's nothing I can do, especially not now after so long. 

I feel like telling her would just make it worse but I hate having her think I'm just a shitty friend. WHAT DO?!?

Can you condense the text into a few key points?
<bot>: friend's bf likes flirting with me, I'm shitty at dealing with stuff like this, now I don't know how to fix it.