Once a cheater always a cheater?

I think that at a certain age cheating is common. I think that as long as there is real love in a relationship everything can be worked out but how do you work out cheating. She is now in her late 30's and wants to try again shes a real adult now she cheated on me once and flirted alot when we were together that was back in our early and mid 20's. I want to work it out but don't know where to start.