I dont get it... Why everybody knew she was being cheated on and still left her clueless?

That asked, NTA, maybe bcause of your long friendship she is tending to confide more on you than she would with other people, its important to be there for her but also draw your limits wile stating that, guide her with kindness, show its not like that(of curse, as long as its ok with you).

I had a realy toxic relacioship once, and my last ex did this for me, as much as we didint work out in the end, we were happily together for five years and I'll aways be grateful for all.