People make errors in judgement all the time. She fessed up to her error w/out being prompted. If it’s a one-time thing because of whatever insecurities she may have had and he forgave her, then mazel tov to them.
If it becomes persistent, then cut ties and move on. As gingersplke88 said, building trust again takes time.
It’s much like the story from a few years ago where people complained about the horrible potholes and filled them in with flowers.
Story: >