I’m in a situation at the moment where I have continually forgiven my partner for similar behaviour and enough is enough. I’ve come to realise that he doesn’t respect me and I need to respect myself. Maybe she is in a similar situation.
However, I find it strange that she moved on to dating another guy so quickly after you guys broke up. I feel hurt and betrayed by what happened to me that it will take me a long long time to even begin trusting somebody else again. I think your best bet is to move on and cut contact to a minimum as it won’t be good for either of you, dating new people and staying in touch with such a recent ex.