I know I’m not the only writer that does this. It’s an author’s prerogative to put friends and ex friends into their writing somehow, and then mess with their lives. In ways we would never even think of doing in “real life”.
Like if it’s someone you really like, you can have them get married, or have a child, or any number of things that you know would make them happy. And then, because it’s fiction, you can have them get divorced, lose custody and fall into a pit of darkness, booze and self-hatred. Not because you don’t love your friend, but because it’s interesting. It gives them character .
But, it’s also really cathartic to base a character (at least a tiny bit) on someone who has pissed you off, or in my case, break my heart. There are a few directions you can take. You could take the high road: put them in, and give them anything they ever wanted.
You can make them the sleaziest, lowest, ugliest form of human. And humiliate, harm, and torture them for an entire book. Or at least a chapter.
I’m not going to tell what I wrote. Because it doesn’t matter. What matters: I let my art deal with my anger. Therefore there are no 3am drunk dials (well, I’d never do that), errant messages of missing/anger (depending). And that’s what art is good at. Emotions. Anger, love, hate, angst, infatuation, etc.
So go ahead and kill off that ex in your next story. It’s good for the soul, and no one really gets hurt.