Is she in therapy? Nightmares are something I struggle with, as well as anxious attachment. When I'm feeling insecure, I'm likely to have nightmares about my partner breaking up with me, being unfaithful, etc. I think the best you can do is support her in getting to the bottom of these dreams, which she should do with a therapist. I'm curious if she shows other signs of being insecure or anxiously attached? Some examples might be looking through your phone, getting upset when you spend time with friends, etc.