My DB could be your son. About a year ago he found out she was having an affair. He is an amazing Dad, while she is a somewhat inadequate mother - having said that she had only recently gone back to work after a long break as a SAHM, and so there was a massive difference in their earnings. He was on about 100k and she was on about 20k (part time admin role, well below her potential - she was a barrister).
He had excellent legal advice, and so the outcome which I think is grossly unfair, actually reflects the law, and was not the result of poor advice.
His bottom line was that they should both share custody 50/50. They both needed houses to do so - he was not going to live in a shoe box and leave her in the matrimonial house as was suggested.
He ended up giving her roughly 85% of the assets, which included buying her out of his pension. It was complicated - but he had received a lot of money from my dad, which was to an extent protected, but ultimately became part of the spoils. He now has a massive mortgage, and she is intending to get a tiny one - but they both should have houses. He pays a lot of money for the children but no spousal maintenance.
Given the differences with your DS's situation, maybe a more equitable outcome can be achieved? If he is a hands on dad, I would suggest he goes for joint custody, and insist that they have similar sized houses, albeit smaller than the matrimonial home.
Good luck - I know it is a hideous situation to be in.