A problem with wis wife!

[Kostas]

A close friend of mine has a big problem with her wife! He thinks that she's fallen in love with another man so he sometimes becomes very upset. He isn't sure about this, but only in the thought he is going to be mad. He doesn't know what to do. He ask for me to give him my advice, but in fact I don't know what I must tell him to do exactly... Can someone advises him anything to do?