In poly, it is not sustainable to let partners complain about their other partners. It is not ethical, and it is not good poly. I get that OP wants to be that person for her. We all do, right? But in poly you have to set very clear boundaries between the different relationships for it to work.

OP even admits that letting her complain to him doesn't help her at all, and that they are enabling her instead of empowering her. They are right. If they refused to be part of the codependent chain, if they set clear boundaries about this, or even pulled out of this relationship, she might see things clearer, it might even spur her to make changes in her other relationships. We don't know.

But one thing is for sure. OP being the person she bitches to about her other relationship won't make their relationship healthy, and it won't help her make changes in her other relationships.