What arguments are there to convince someone to cheat on their partner?
I'm writing a short story about a world where, when something really bad happens to someone, there's a slight chance they'll turn into a Terror, a monster that's a twisted version of a human. This phenomenon is pretty rare and only started recently, so people still don't take it seriously. Most of the story is told from the perspective of a man who discovers his wife is cheating on him and becomes a Terror. I want to add a bit from the perspective of the woman who cheated.
To clarify, it would be relatively short, just so the reader can understand, at least partially, her actions (even though she cheated on him, she tries to prevent her husband from becoming a Terror, risking her life in the process). So I thought of presenting only the conversation that convinced the wife to cheat, and later, how she experiences, from her perspective, the husband (that she still loves) becoming a monster because of her.
The idea is that it's a combination of self-sabotage (she doesn't believe she deserves to be with him), the fact that they've been together since high school (she doesn't know who she is without him), and a toxic friend who convinces her to cheat. I have thought about the kinds of arguments a manipulative person might use, but I have a hard time understanding this way of thinking, and as a result, I'm not sure what arguments she could bring to such a situation.