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Jenni855 OK, so you've been separated for more than a year. Personally, I think people should not start dating, as in for the purpose of finding a partner, for at least a year after their divorce is final.
Introducing your daughter to his new GF, when you have not divorced, is a serious error of judgment on his part, but should not really have a negative effect on your daughter, unless the roles have not been clarified. His GF is not her mother, you are, and she has no role in raising her.
If you're negotiating custody issues, you may want to pay close attention to the kids' situation, vis-a-vis romantic partners. Make sure your daughter understands that you are her mother, and that any time she spends with her dad's GF, alone, is at YOUR discretion, and make sure he understands that.
Also, make sure he understands you're doing this out of concern for your child, and not to spite him (or because you have not gotten over him). Right?