I will empathise with the husband a little bit in my post however I should make clear his behaviour is unacceptable and you are right that you cannot go back.

Eight years is a long time and it is natural for eyes to wander in this time and there to be bad events like this. However as I have said to act upon this is unacceptable. When that this relationship ends and he will realise he has thrown away the longest relationship of his life and has now doomed himself probably to dating in his thirties and finding a partner like that, I am certain he will feel very big regrets.

We should remember that he is human and though he is a very bad person for doing this, from your description it seems he has accepted temptation and demonstrated himself as having weak character. That is the worst fault here I think. You can view it as natural selection that this idiot partner and his genetics have ruined a good situation for himself and traded it for something that is st its best uncertain.