One thing is that all parties need to work in a relationship and a completely different thing is to expect that includes forgiving anything (not just cheating). They can decide that it is not working anymore and break up. And it is fair.

That idea that just because you are in a relationship when the other cheats you should put the same work (or more) than the cheater to fix things is unfair to the person who has been hurt and betrayed. Very selfish on the cheater part, since it expects that the person the cheater has hurt keeps working on a relationship that has been good for the cheater but not so good for the cheated-on person.

That is not how things work. All parties involved need to decide on their own if the relationship is worth it. And sometimes the answer is no.