I agree. All you can do it tell him. Let him know you don't wish to interfere, and you will only say this once,
but it extremely important, and you see him heading for destruction.
He won't listen of course.
And this is the reason you should only say it once:
When you step in between lovers, you will become the enemy of both of them.
He may say you were right later, but that means nothing.
Then again, it is possible he could be happy with his new love.
But you might want to redefine honor and integrity for a man who allowed himself to fall in love while he is married, and is divorcing his wife for a younger woman.