"Your boy" should leave his wife if he's really that unhappy and there's no spark left at all and he truly feels he can't get it back. I couldn't live a life like that.
But your boy is also an idiot to think things are going to last with the blonde. There are people who cheat and people who don't. I'm sure he and the adulteress feel right now they're madly in love and always will be but that's not real life and I'm willin to bet when they're no longer sneaking around their spouse's backs and they give it 6 months to a year, they're going to be in normal love and it's going to be same ol same ol.
Then one of them is likely going to cheat. Not tomorrow, but eventually.
I could never be with someone who would cheat on a spouse because it's unacceptable in my book. Anybody who'd do that doesn't hold marriage in as high regard as I do. If you would cheat then you will cheat. Simple as that. Ew to your "friend" and this woman.