Three principles of relationships:
#1.) We are NEVER upset for the reason that we think. ( it just seems that way).
#2.) Light always dispels dark, in other words, a fantasy has appeal BECAUSE it is a secret. When the secret is brought into the light ( peacefully !) it loses it's magnetic pull. A gentle conversation ( along w/ prayer) will bring this 'problem' to the light. The problem is they are both afraid of intimacy. Intimacy is trusting that what you think, feel & desire is true and can be therefore shared w/someone else. She is afraid to share her thoughts, feelings and desires with her husband AND her 'boyfriend' She thus attempts to withhold 'The' Truth from both of them. (It doesn' work) He is afraid to talk honestly with her, and therefore knows within his mind that He is not sharing his thoughts, feelings & desires with her, and he therefore (incorrectly) reasons that she doesn't want to share her T,F & Desires with him. ( This may have helped her go share SOME of her T,F & Desires with someone else ... in this case her girlfriend. She has one set of T, F & Desires but attepmts to 'control' them by sharing them in pieces with 3 people, rather than with any One person.
#.3.) We never judge others for their 'sin', but rather, only for our own ( which we inadvertantly project onto them. IE - your friend needs to 'forgive' his wife -- and come to truth about what HE really wants. He is not guilty of some sin himself, because He doesn't even realize ( yet ) that his 'anger' or upsetness with his wife, is in truth, a projection of his own judgment of himself for something he did in the past. When he forgives her - for what she didn't do to to him ( but seemed to ! ) he will find inner peace. That is probably the essence of Him that she is missing ! His carefree, loving, creative self.
I realize this may sound weird, but it is true and will save him more time than he can possibly imagine. The truth is they are both innocent. They both want to be able to express their LOVING thoughts, feelings & desires. They each desire this. In this relationship - now. How do I know this ? Simple - I have been there, done that and got the T Shirt. Don't let him take years to resolve this. It can be healed right now. The result will be inner peace for both of them ( unless they actively DON"T WANT peace) AND a great True relationship - either as partners / spouses / lovers, or they will release each other from the illusion of being in a happy, loving, peaceful & contented relationship, then go enjoy it WHOLLY with someone else is is ready.
Best wishes !!!