I think he should of told his son about the affair.
I don't know about you all, but that is how I feel. If my mom or dad had an affair with my current boy/girlfriend then I'd like to know that, so I could make my own decisions on what to do, rather then to keep everything 'peaceful'. It's just horrible if you were to ever find out about something like that, but it is better to hear of it sooner rather than later or - in all honesty - to never know about it at all.
I hated the last part when she explained to the father that she loved his son, and that in a way the son was like the father, it didn't make me feel any better about the situation at all, I don't know if it was meant to make you do feel better about it, but that is just how I feel with the whole thing. What's your opinion?