I'm sorry for the situation you're in. Nothing feels worse than feeling like a rebound or second best with your SO. The most concerning thing is that she has actually said she would get back together with him if she had the chance. And that she talks about an ex with her friends more highly or often than her fiancee based on how I read it.

If conversations aren't working would she consider couples counseling? Because you should absolutely sort this out before the wedding date, push it back if need be. She needs to decide if she cares more about her ex or you, and if the answer is her ex then she should not be getting married to you.

How would you feel if you got married and this continued? It's your feelings that are being ignored, and can you even say it's jealously if you know your fiancee would dump you for someone else? What would happen to your current relationship or your marriage if this ex showed up again? It certainly doesn't sound like she would reject him if he wanted to start over.