The issue is to regard "chain of interpretation" as "chain of infallibility"
A man by primal instinct is a builder; he will fix all the broken, dented, wrecked things until they are functional. Apparently the same instinct doesn't apply to when a woman is involved. If there's an unsatisfactory element involved within an existing marriage, it's best to work on it. We're humans. We are malleable. We evolve. Remarriage will only further bury the current relationship he's not very fond of. Don't give me the "he's happy with family" nonsense.
I would not object to men marrying again; so as long you do your due diligence via integrity and maintain your partner's dignity. How to maintain her dignity? By letting her know of your plans so that she too, can make a decision for herself.
If you think women [after having spent years with you] can't notice behavioral changes in a man, think again. We women know you inside out. He sure is naive for assuming he can keep marriage number 2 "under the table" without the current one finding out.
Lets put aside the fact that remarriage without involving/informing pre-existing wife would be complete fraud and emotional abuse. Remarriage is a major life event and an utter disgrace to the spouse being blindsided. No woman deserves this and far less if she has birthed your children. This friend of yours should consult with his wife and keep her in the loop of his plans. Being heartbroken by brutal honesty is far better than being heartbroken by deception.