In counselling, reality is what the person thinks it is.
Before you have a heart attack! - what happens is, as that person feels heard, their 'reactivity' comes right down and they are then open to considering 'their part to play'.
The trouble is, as you have described, it is very hard to hear and you have to manage your own defensiveness that is a natural response, which then triggers more reactivity!. But if you can? The reactivity goes down and people can meet in the middle.
But ultimately, you are right. At the bottom of all anger, is grief. Grief that life happened when people were making other plans. To constantly vent is to deny the reality and say 'I don't want it to be like that'. You will never be at peace all the time that is happening.
But it is important to process things. I have heard 70 YEAR OLDS be bitter and twisted about long dead people and long gone events. That really were traumatic/unjust/unfair. But they still have power over them because they have not been accepted. What is the use of that?
So I would say, go with the counselling, her reality is not your reality, stay calm, don't react, and maybe she can start moving to a place where she stops blaming and starts taking control of what she can control - herself.
Caveat: you have to have a really good counsellor. What is this person's qualifications? You are entitled to ask.