Your friend thinks he is going to walk away from a bad situation and start a new wonderful life with the blonde. It WILL NOT work like that.
You cannot go through a divorce after a long-term marraige; all the guilt of breaking a family apart; seeing everyone's tears and the backlash from the kids...and be the same person after it's over.
He may be better off divorced, but it isn't going to be the easy way out he's thinks it is. It's pain, pain, and more pain for everyone involved. And chances are he will get back together with the wife and try and work things out at least once. Where will that leave him with the blonde?
Whether or not he wants to end his marraige should have been decided before; independently of a new relationship. Now it's just a mess.