Your friend sounds very narcissistic, but that doesn't excuse the husband from cheating on his wife (not that that's necessarily what you're focusing on). If not breaking up a family is so important, it shouldn't matter whether his wife had become an unattractive SO after having the husband's children.
The wife as well as the OP's future should matter to them. I'm not discussing fault, excusing anything, etc. Literally, not the topic I'm discussing. Placing blame won't change the future for someone cheated on; self reflection can.
And it's quite possible the wife and/or the OP (the 2 people cheated on) might self reflect and discover they feel they couldn't have done anything better in the relationship (and/or better for themselves, as the OP has already reflected on, he understands now he should have left earlier and saw signs.)
The point is that it's important to examine your role in situations. It's not about fault, it's about the role you played.