In most scenarios, it's good practice to not meddle in an ex's new relationship. Regardless of intentions, it can make you look desperate or vindictive, plus you really can't know how much someone has changed in a new relationship.
However, if your ex was an abuser - emotionally or physically, alerting their next partner can save another person from months or years of pain. The only issue is, approaching an ex's partner to tell them the truth can be an awkward and tricky situation to navigate.
AITA for warning my ex’s boyfriend about her?
I dated a really horrible person who was verbally abusive to me. She would constantly say things like i wasn’t good enough for her and she was never there for me when I needed it most. Before we broke up she would go through periods of several days of just ghosting me and she even made a post talking about how girls should ghost their boyfriends to make them miss them.