No relationship ends without some kind of emotional upheaval, even when it's for the best. Your friend will see that eventually but it takes time. Right now she's probably scared of what the future holds and guilt-ridden about leaving, however unjustified that guilt is. People grieve when relationships fail and they reflect on the "if only" or "should have been" or " did I do the right thing" type thoughts a lot at the start but that will ease.

Right now the best thing you can do is let her talk when she needs to, without judgment, and let her be alone when she doesn't want to. Be of practical help to her- help her find a place to stay, a job, help with kids if she has them. Try think about what's weighing on her mind most and ways to alleviate that. One day the feelings will change to ones of relief that she got out of it. It won't happen overnight but it will.