a) going through a breakup with another partner
Tell her you are sorry she is going through this. How could you best help at this time? Because...
b) usually shuts you out instead of turning to you
So if this is case, you will just give her space unless told otherwise.
c) blames you for their breakup.
Today, she suddenly raised it with him and ended it.
You do not control her behavior. She controls her behavior. She ended it. She did not have to. If she is upset at herself for ending it, she could ask him to come back together rather than blame you.
But you? No, thank you. You will not hold the blame bag on that one. So not up for more talk on that. You don't have to take blame baggage. You could tell her you do not appreciate it and won't have it. End of discussion.
d) is hostile, cruel and confrontational through their pain
Have you told her you do not want to be treated this way? Does she make reasonable attemps to stop it? Where's your limit on this behavior if it continues?
Are you at limit? If so? You either accept her this way and she will always be this way. Or you walk away because you rather be without this behavior.
Again only she can control her behaves.
You can control yours. You choose to stay in the line of fire? You choose to go? That's all up to you.