This happened to me recently and still going through it. Chances are it hasn't been good for awhile and her mental health is probably triggered by him for all you know. That was the case for me. But he also was abusive etc so maybe different.
At the moment she is in the stage where she will be panicked, feeling abandoned and will be worrying how she will get by without him and perhaps even in denial it's really happening.
I'm just over two weeks in and now starting to feel the new normal.
Best thing you can do is be there for her to vent, give logical responses to help her not panic and think of the worst outcomes.
Perhaps even research what she may be entitled to etc or how she can get in a better position financially. As right now the second stage is acceptance and realising no one is coming to save her she has to be the one to do it.
So if that means getting a new job, more hours, a new hobby anything she has to start thinking and taking these actions.
The more safe she feels in this new situation the better and quicker she will bounce back.
I've lost a stone in a few weeks it has been brutal as my partner just left and it sounds similar to her situation.
Strongly suggest she gets a therapist asap as she needs to have a space to unpick what's happened and how to move forward.