She "made some mistakes".
That could mean anything... She could have been fucking this guy for a while behind this guys back... and he might even have known about it then and brushed it off... then this came around, and it was straight obvious what was going on, and he realized that this girl wasn't one bit who he thought she was.
It takes a lot to do what he did. Trust is obviously HUGE on his list, and by bursting such a huge trust bubble, after 5 years, he obviously felt she deserved this treatment.
She's not going to be a stable person for a while after this though. Jesus christ...