Back before my divorce, ex's daughter was a handful. She would try her hand at lying about little things. So in a fit of desperation I installed a parent app on her cell phone. She let a boy convince her to meet him at McDonald's for breakfast before school. I let it play out and since I worked 3rd shift, the morning arrived and I took off early. Beat them to the Micky D's and parked in the grocery store parking lot and hid in the dining room of the restaurant. They both arrived, kissed in the parking lot, headed in and went to order. I strolled up behind them and waited and listened, when they had incriminated themselves, ordered and he paid, I spoke up and told the cashier they weren't going to be needing that order but you can keep the money. I walked them outside and to his truck and asked him exactly what he was planning on doing? I also reminded him that I was behind them the entire time and to not lie to me. He sheepishly told me and I told him that if he came around my daughter again that his parents wouldn't find enough to bury and to make sure that he let's everyone know exactly how unhinged I am. Needless to say, I never had any issues after that.
Thankfully, she is no longer my problem to deal with.