Under the circumstances I would not be shocked that your son's marriage is coming to an end. In fact in the U.S. the divorce rate hovers around 50%.
Given the fact your son was only (17 years old) and got married due to an "unplanned pregnancy" not many people would have bet a ton of money on the couple living "happily ever after".
The odds were against them!
Money does not buy happiness. Women cheat on "rich husbands" all the time and men cheat on "beautiful wives" all of the time. You never know what makes someone "happy" and oftentimes they themselves don't know. Happiness is a "feeling" not material.
There are lots of people who have all the "trappings" of success in their lives but go to bed (feeling depressed) every single night.
They realize there are lots of other people who would gladly trade places with them but still that knowledge does not change the fact that they're unhappy.
Generally speaking women cheat with men who make them (feel) special. It's possible your son made climbing the corporate ladder to provide for his family a bigger priority than what (she) needed or desired in order to feel loved. I personally knew of a man who was working two jobs to provide the best life possible for his family and while he was doing that his wife was partying with girlfriends and eventually started having an affair.
If two people don't share the same values, want the same things for the marriage, or (naturally) agree on how to obtain those things then the marriage is likely to fail.
His priorities were not (her) priorities.
It's only natural for you to sympathize with your son. However as the grandparent your top priority should be maintaining a healthy loving relationship with your grandchildren. In order to accomplish this you may have to bite your tongue with regard to the choices of (their mother).
Eventually they'll figure things out for themselves.