My husband has a liaison with another woman. He has been diagnosed as suffering from a Narcissistic Personality Disorder.
Narcissists are people who fail to maintain a stable sense of self worth.
It is, therefore, nigh impossible to alter this particular behavior trait in a somatic Narcissist. Sexual interactions serve as a constant, reliable, easy to obtain source of Narcissistic supply. Ideally, all contacts between your spouse and his lover should be immediately and irrevocably severed. But this is usually too much to ask for. So, you should define when is she allowed to call, whether she is allowed to write to him at all and in which circumstances, what are the subjects she is allowed to broach in her correspondence and phone calls, when is he allowed to see her and what other modes of interaction are permissible.