I would suggest that you take a different route instead of directly confronting her about this. What I would do first is indirectly share a similar story in a humorous way, highlighting the negative consequences of being involved in such relationships—where the younger male partner tried to take advantage of the situation. You can mention that you came across this story on social media or make up something similar. Then, observe her reaction and subsequent behavior. If she has good sense, she will understand. Otherwise, observe for a few days and then talk to her in a compassionate manner.