I think it depends what he learnt from the experience, but critically, what did he learn about himself? Describing the OW as a psycho is an enormous red flag, as is his weakness at not ending the relationship with her "because he didn't want to hurt her". If that's still his character, what that means is he lacks moral courage, won't tell people how it is, puts his head in the sand and shuts his emotions off when life becomes difficult.
I'd ask lots more questions about the way he currently deals with difficult situations in life, because what I've described above usually transcends someone's personal life and you will often find that in all sorts of other situations, the person is a "people pleaser" who will never be straightforward and assertive, will shirk responsibility and blame others, or will let a situation get out of control until it gets too big and messy to deal with, so he runs away.
I don't agree that once a cheater always a cheater, but I certainly don't think that someone's propensity to be unfaithful is governed by the person they are with - unfortunately people in your shoes will often protest that "he wouldn't cheat on me" which is often the triumph of hope over experience.
IME, it's much more to do with whether a person has learnt to be different in all aspects of their life. It can be helpful to depersonalise this and ask him whether he feels that infidelity is ever justified as a behaviour choice for people generally. The people who are most likely to give themselves permission to be unfaithful are those who think that sometimes, the end justifies the means and all's fair in love and war. Whereas the person who cannot think of any justification for deceiving someone and acknowledges that there are always other choices, is the safest bet.