I understand that jaw...I am not sure how relevant that is after the time past except for her to find out what you already told her...however it may be...I just wouldn't react to her spending time with her. Perhaps, she is getting information to confirm what you state...that to me is positive, maybe very positive. Agreed perhaps odd but I would definitely not react negatively to it. Won't help you. Also, try to remember that she said that she wanted this b/c of this woman, but you know as well as I do, we all got a long list of "reasons". It could be that she is losing this "reason", that is positive...remember WA's look for reasons to validate their choice. Just don't see her being best friends with this person, seems like information exchange to me. Blah blah.
I still think it is interesting that she took kids. I think she wanted you to find out. Besides, she wants to spend time with you. Don't push. Be happy with what is positive and enjoy that time.