I wonder if the title phrase is ever used in a meaning different from the obvious one: cheating in a relationship? Can it refer to cheating in an exam or obtaining money in a dishonest way?
The "Once a, always a" is a common saying and can be used to illustrate multiple situations:
once a ——, always a ——
proverb: A person cannot change their fundamental nature
once a whinger, always a whinger
So yes, you can use it for other types of cheating as well.