Firstly it is never, ever better to pretend. EVER. So let's kill that thinking right now. Secondly, screw the father's feelings, the wife deserves to be freed from him (not only because he's cheating, but if he is gay, then it's not fair on her to be tied to a lie). He must, at the very least breaks it off with his wife. Her feelings in the matter are just as important. Your friend needs to understand a family apart but happier is always better than a "none broken" family held together by lies and empty fulfilment.