Man that is a FINE line there.....even if it IS true.
Truly this is walking on a razor blade.
I would be very cautious about how and what was said. If you have to go there make it about the kids as much as possible. For example: "S9 has shared with me he feels like you are taking sides against him and I have seen a couple of things that perhaps you could clarify so I can help him understand." If she says she'll do it, you could make the point that you could be more helpful to him when he talks to you about it, if you understand where she's coming from.
