Tomodachi Life fights, jealousy, breakups, ex-friend events, and divorces all have randomness. You cannot press a button and make A and B definitely fight, and you cannot guarantee that one couple will definitely split up.
What you can do is raise the chance that relationship events have room to happen. Think less about one trigger, and more about building an island with more Miis, more personality variety, more active relationship lines, more shared spaces, and fewer instant repairs.
If you are trying to protect a relationship, use the fights and making-up guide instead. This page is for players who want to watch the island get messier.