This is a really tough one b/c as a parent I would want to know if my child engaged in dangerous behavior. However... you could be opening a can of worms here- and potential litigation. Unless you know these parents VERY well, I would deal with punishing your own child and address your ex regarding WHY she was unsupervised/unaccounted for long enough to pull this off.
My sister pulled this one at the same age. My parents were taking me to college and left my sister with their best friends for the weekend. These are mature (older than my parents) adults who raised three children and are very responsible. They asked my parents "what activities" my sister was allowed to do, curfew, etc. My parents told them the "rule"- in front of my sister... she worked around them. She was allowed to go to the first football jamboree and out to dinner, but had to be home by 10. Fine. She went to the football jamboree and then went to my parents' house with her friends and was back at their house by 10. The neighbors ratted her out.
Needless to say, she wasn't left home again AND was grounded for a month AND had to be home an hour after the Homecoming dance ended.
Punish her- consult with your ex (good chance she told him that she was out with her friends) and let the other kids see/hear that you know what happened and that your daughter was punished. I would also tell your daughter that IF something like this EVER happens again, you will contact each parent and fill them in b/c it will NOT be tolerated in your home.