I'm so sorry chicken x
Your friend sounds like a good one. Despite the initial reluctance and confusing messages.
My ex DP of 15 years was fucking my teenage brothers best friend (not a teenager but not far off it) for over a year. Ex DP was driving regularly over 200 miles to see her on weekends when he told me he was busy with work/family/hobbies.
My family who live in the town all knew about it, none of them thought I should know.
She got pregnant which was the final straw for him and he fucked off to Brazil for 4 months with no explanation for me, just text me from the beach saying he was having some time out. I had no idea what was going on but it signalled the end for me. Broke up with him by text a few weeks later with him still abroad.
It all came out what he had been up to when I was asked by my 78 year old nan a few months later why I was letting my partner fuck other women.
My sisters used excuses that A) they didn't know, B) they knew but only for a short time, C) they didn't want to hurt me, and D) they didn't want to take sides (what sides?)
My own brother introduced the two and apparently watched them flirt. One of my sisters saw them driving around in the early days of their courtship (which she denied but her partner confirmed they had), and my other sister knew about it for months and didn't want to take sides between me and Ex.
Things with my family have never recovered, I don't want anything to do with them and whilst I love them, I absolutely do not like them.
I hope you stay strong, and keep the good ones close to you.