If this person is a really close friend, point out to him that he is most likely addicted to the feelings this new relationship is giving him, rather than the OW herself. He's also probably re-writing history right now, pretending that he'd been unhappy for years. If you knew him before the OW came along, you can remind him of events and conversations when he was telling a different story.
You could point out that these feelings are very unlikely to last and what will he do a few years down the line when someone else turns up and starts to give him an esteem boost - repeat history? Likewise, the OW might meet someone else too - and since she is younger, will probably have more opportunities.
It also depends what matters to him. He is likely to have a very strained relationship with his children, who will have divided loyalties - and there is likely to be an impact on their education and perhaps in the relationships they go on to form themselves later on. Plus it will be financially very costly for him.
I mention his wife last because no doubt he is de-humanising her right now, but ask him to imagine how he'd feel if he finds out in a few months time that she is sleeping with someone else, which will be her right. And eventually, how he will feel if another man is playing an active step-dad role.
Do say something. Your friend is in the grip of a type of insanity at the moment and he needs to realise that relationships that start this way, statistically last a shorter time than the relationship he is leaving.
What you perhaps don't know though is whether his wife would even have him back, after all the hurt caused.