Most Helpful Girl
It's may be a contributing factor. But it doesn't excuse the action. I think it's important to understand that sometimes mental illnesses can cause someone to act out like that, and if you feel like you're equipped to deal with it and be with the person then that's great. But you're also not obligated to stay with them if they're acting in a way that makes you uncomfortable or in a way that you don't know how to handle.
Most Helpful Guy
Cheating is a choice, so there's no need to try to find excuses for what happened.