Such a case is a symptom of minds that are not well trained, and stunted spirituality. Even if the man gets the other woman, there's a great likelihood that all will end up the same way as now. If the wife leaves him, there's no guarantee that life will be any better for her. It's a lose-lose situation.
The solution involves seeking enlightenment and enduring and transforming the situation as it is. How to do that is a matter determined by the spiritual hunger and effort you are willing to exert, and the training available. Getting help from a professional counselor may be a good place to start since he/she will help you open your mind to the dynamics of your relationship and you'll be able to make better decisions.
A pet theory for me in such cases is 'object relations'. Here one gets to explore how a person chooses a spouse, the roles that he/she projects on the spouse, the responses elicited, the emergent needs and the like. That way behavior can be mapped and even altered. But that's just a start. But a good start in a life long journey.