He should do what he thinks is right. He can walk away, we have non-recourse loans in California. But to blame the bank is just silly.
Blaming the banks is directing blame towards the wrong entity. These borrowrs should focus blame on "mortgage brokers", and at themselves for being such poor shoppers in the first place, for a variety of reasons.
Almost anyone - except those who couldn't get a loan anywhere because of a record of bad credit - could get a mortgage loan without any fee at all except for normal closing costs (appraisal, legal, recording fees, title insurance) at a good fixed rate for about any term from 15 to 30 years.
The brokers (still) add on handsomely for a service of questionable value for what local banks do very well and for virtually no charge at all. Writing loans is something banks see as just a cost of doing business, so there is not charge to customers.
These brokers got anywhere from between 3 to 12 thousand bucks per loan, a substantial charge added into the loan from the beginning, and then walked away, usually after having done nothing that contributed to the deal except to complicate the closings and the lives of the borrower while they screw around getting the best situation for themselves.
Many times a loan is closed with the seller waiting overnight for their funds to arrive because the mortgage bank wants to examine all documents before paying up; the seller not having had a choice in the matter.
People are too smart for their own good. They are so convinced of their own cleverness that they have and will continue to go to these so called mortgage brokers. Part of this foolishness is propogated by the internet, and the fact that they get hooked up with a broker online, preferring to avoid personal contact.
The case of someone being underwater at 50% valuation is a rare case. More usual is a marginal amount of perhaps 15% which will be easily regained during the mortgage's term of payment.