Although I am against the aspects of lying and dishonesty that go along with infidelity, I have to say that I'm with Dan Savage - it's not good, and it's also not the absolutely worst or most unforgivable thing a person can do. Americans tend to think of it as an unforgivable offense - all the while 35%-60% of Americans are engaging in infidelity. I hope your son and his wife work things out - that is if that's what they want.

"Treating monogamy, rather than honesty or joy or humor, as the main indicator of a successful marriage gives people unrealistic expectations of themselves and their partners. And that, Savage says, destroys more families than it saves."

https://www.nytimes.com/2011/07/03/magazine/infidelity-will-keep-us-together.html