Unfortunately, it sounds like this guy is a very clever bully. He laughs. He's not using any profanity or hate speech, or taking any action other than laughing. So if he were to be reported, what would someone say? "I want to report him for laughing". Obviously, that would go nowhere, because laughing isn't a crime. We're told to "laugh it off", and that "laughter is the best medicine".
So he's figured out how to get under your daughter's skin by using a very socially acceptable tactic.
I don't think that the usual responses are going to work in this case, because it's very clever and disguised bullying. In fact, that might be what is so troubling to your daughter. He's not calling her names, or hitting her, or trashing her belongings. He's laughing, and that's easily disguised or explained. She can't put it into any context because after all, laughing is supposed to be good.
So I think a whole new approach might be helpful. I have had to deal with a sociopathic relative who is not outwardly and apparently cruel, but who uses very clever, subtle ways to cause great harm and emotional injury. He doesn't attack with words, but he uses very clever ways to cause the other person (his victim) to doubt themselves, to doubt everything, to have real fear. It has been helpful to view what he does as a clear statement about his own evil self, and not anything to do with anyone else. When your daughter's ex laughs loudly as soon as he sees her, he's not laughing at her, he's projecting his own sad sense of worth and demonstrating how poorly he communicates and how little he has to offer. He can only laugh. He can't deal with situations like an intelligent, caring person. He just laughs like a cackling crow or a hyena. Maybe if your daughter realizes that, it might help her to ignore him.