Don't challenge her relationship in any way, or make suggestions on what she should do.Be there for her when she wants to talk, and that's it for now.
This is exactly what you want to happen, and it's a necessary step to making her want you back.
Some time after you've disconnected from your exgirlfriend, she's going to start poking around for you.
Hearing all about your ex's new boyfriend may be painful, but it's necessary.
Consider it recon: you need to learn your enemy, spot weaknesses, and find the holes in the armor of your ex-girlfriend's new relationship.
Even if she was just bashing her new boyfriend for something he did, suddenly she'll be defending him against what she perceives to be an attack that you started.