We don't know that Matt has cheated or done the dirty and granted we don't know he hasn't either
Just to play devil's advocate I woke up one Monday morning after 10 years together and announced I was leaving and I didn't want to be with him anymore. It was out of the blue, he was devastated and I felt like the worst person in the world. There was no one else involved and I'd never cheated, it was also one of the toughest decisions I've ever made.
Surely you don't advocate a couple staying together if one is unhappy because it may devastate the other one ??
Just reserve judgement until, or if ever, you know the full facts, things are very often not as they seem to appear.