I've been a stepmum for over 10 years, I personally don't think stepping on them BM's toes while she's already aggravated will help matters. If you avoiding going to pick up will help reduce the conflict between the parents, be the bigger person and don't go. You have this problem here because the parents can't cooperate to co-parent. If your partner goes up to ask her even very nicely why the child is saying this, she's definitely going to go defensive or may even counter attack. Kids this age can view things differently, being "mean" to them can actually be them being sent to bed without a second bowl of ice cream or being made to eat their vegies. It can be easily taken wrongly. I don't see a way out of discussing it with her though, if you're genuinely concerned and can't discuss it with her, DOCS can be involved and they can investigate.