She is not be respectful towards your relationship at all. The fact that she has been forgiven for cheating in the past and you have since tried moving forward should really have had a bit more of an impact on her behavior. Unfortunately, it seems that it hasn’t. Flirting isn’t cheating but it could be considered a huge breach of trust in the same respect.
I think your best bet is to try to have a conversation with her about it. When she isn’t being little and you aren’t angry or upset. Sometimes our emotions can really could our judgement and it isn’t a lot of fun trying to talk to our partner that way.
Depending on how she responds here are some next steps that you can take:
If she is mocking and unapologetic: You may need to take a break from her. Ask her to evaluate the value the relationship that holds in her life. She needs to value you and your relationship, not be disrespectful towards you to her friends or anyone else for that matter.
If she is apologetic: Make sure she understands that it is up to her to build up trust with you. You should not have to be hearing things from other people or watch her flirt with other people. She may be hurt by this but if she really cares about you she will work to make the relationship work. Let her know that you are supportive and still care, but you have feelings as well.