Should a woman, leave a fun, caring and loving guy and go back to a man that slept with her for years but never made love to her, then left her to live, sleep and make love with another woman and only returned to her after he saw her happy with someone else, even if they did have 5 years of history?

Why would a woman let go of current happiness to go back to a lifetime of despair?

History is just that, history. It takes a past to teach us a lesson in right and wrong, what is healthy and unhealthy.

A little more detail please!

Are you this woman or her husband/partner?

How do we know that this guy you speak of is really the warm and caring, fun and loving guy you stated?

Not to sound cynical, but...

Generally speaking, I do not know why a woman would leave such a great guy for someone in the past who mistreated her. Maybe she likes that kind of humiliation, maybe she wants to know for 100% sure that he was and still is someone who is best left alone.
Maybe he changed and maybe she changed.

If you are the guy who has been left, then say adios and bon voyage to her. Let her find her way. That does not mean you have to be there to pick up any pieces.

Sounds like an unhealthy confused person, who is a glutton for misery and pain.