Would she be willing to read here a bit? Maybe the two of you could read together. That way a question might evolve where by we all can give you some guidance and support in a specific way rather than a broad way.
Maybe if I ask some questions it will help.
What makes you think this ex cheated rather than was poly?
What steps have you made to walk through your jealousy, or her through hers?
What part of poly does she think she is successful at so as to make her think she can "do it?"
Anyone want to be friends on Facebook?
Send me your name via PM