Cheater!

[GETTINGLIKEMYMOTHER]

Since I’ve just seen it yet again re infidelity - the word for the perpetrator is cheat!

It's the other way around; someone who commits infidelity is a "cheater". "Cheat" exists as noun in English, although when used for a person it is typically considered archaic, but "cheater" is the normal and established contemporary term for a person who commits infidelity.