It is a very tricky, sensitive area, and kids know that divulging secrets means that they are being 'disloyal'. However I don't think it is ever OK for a child to be told that they have to keep secrets, from anyone or to anyone. So I always teach my kids to tell me if anyone ever asks them to keep a secret.
I wonder whether that is the way to tackle it. Not to specifically hone in on the other parent, but more hone in on that child, keep them wanting to trust both of you by being consistent and safe. Maybe you could just generally chat very casually about friendships, ask who her friends are at school, and how it is all going. Support her general wellbeing and give her another version of what is OK, ie not keeping a secret, not ignoring other children, making good friendships by being a good friend yourself etc. There are books that can introduce the topics in a good way and give your step child another framework.
My DP and his ExW tolerated secrets and their kids kept secrets from them, I could never understand why no one ever tackled this issue. At least your DP seems to have broken away from unhelpful behaviours! My DP left a lot of the 'social rules' to his ExW.
So I bought it up with my DSDs and said that I did not want any secrets in our house. Privacy of course, and a bit of healthy space, but not big secrets.