As others have said, if a woman says that something unpleasant happened sexually, I'm going to believe her, because generally, women don't lie about it. Can they lie about it? Yes, but it's vanishingly rare. The fact that the perpetrator admitted it in a letter as being 'unforgivable' is also fairly solid evidence of male wrongdoing.
He may have felt it was 'unintentional', but that doesn't reduce the impact on the victim. I also can't blame her for not going to the police or uni authorities, because both of which are well known to be shit with dealing with sexual assault.
She's also entitled to confide in her friends. Her friends are entitled to support her and withdraw their friendship wherever they see fit. Honestly, it's actually kind of refreshing to see men taking a stand against sexual misconduct, so the idea that the coroner should be speaking against it is pretty disgusting.
It's sad for the lad's family, but it sounds like his suffering, like his death, was self-inflicted. It's awful to try and lay the blame at the feet of the girl and her friends.