I was on the other side of this - I was the ex.
How long were they together?
How long have they been broken up.
As a rule, when things are going good in the reltionship the woman does not feel the need to talk to her ex - but when things start to go wrong, or she loses interest, she will want to talk to the person she has a close connection with.
I'd imagine they had been together for quite a while.
My ex started talking to me again, telling me all the negative things about her new guy.
My advice would be not to put any conditions on the relationship. Let her do whatever she wants.
Just so that she knows though - that her actions will cause you to react. You will then start talking to other women.... She can't have it both ways.