Infidelity

A friend approached me the other day with a problem. About 12 months ago his wife embarked on an affair with another man which lasted about 6 months. The affair is out in the open which is fine. He and his wife haven't had sex since the end of the affair. They get on well even now and comfortably life in the same house with their children and provide a loving atmosphere. My friend does not want their relationship to end. He would probably not mind if his wife embarked on another affair. What is important to him is that his children have a happy, 2 parent home which they do at the moment.

His wife has just approached him about a resumption of a sexual relationship. He is not interested although in all other ways he feels close to his wife. By saying he doesn't want a sexual relationship any more he puts home stability at risk. By saying 'yes' he crosses a line he is not interested in crossing. What should he do?