I think I might be in the minority here when I say I don't think what she's doing is a problem. In my opinion there's nothing wrong with hanging out with an ex as long as it remains platonic. I understand the insecurity that may come with that; from what I understand it's pretty common, especially in monogamous relationships. That feeling can be difficult to experience and even more difficult to process. But I've found it's less about what's happening and more about your own stories about what it may mean or what you imagine may be happening. The key is to fact check what's really going on. If it's truly platonic, there's really nothing wrong with it.

I would say your friend is being a little inconsiderate, given how rocky things are with her partner. However, while it's good to consider your partner's feelings about something, in the end it's his responsibility to overcome his trust issues and make peace with his fear of being cheated on. It would be nice if she was supporting him a little more as he works through those feelings, but that doesn't mean she needs to alter her behavior just to make him feel better. He needs to do some emotional work of his own.

Overall I would say yes, this sounds like a dysfunctional dynamic that definitely needs work. But ultimately no, I don't think what your friend is doing is wrong. I do think she should examine her reasons regarding escaping the stress and sort that out, but that doesn't make what she's doing wrong.