First, he needs to factually establish the likely selling price of the house (Realtor’s market analysis based on comparables)
The Realtor can also establish a market rental value for the house. If he can rent the house with an option to buy in 1-2-3 years, maybe that is a solution.
but he cannot just make an “assumption” that the house cannot be sold or rented
If market value is low, then seek bank approval for a short sale with no recourse. If the bank is uncivil, then his options are to get legal advice about deed in lieu of foreclosure, documenting that he tried to work with the bank
That is what a civil person would do, imho
I think what happened here, in my opinion, is he bought a house at an over rated value and the bank gave him the money for it, and not it is not worth anything what he actually got a loan for and is probably way way over his head and would still stand too lose thousands if he sold it at a reduced price, and still be in the hole.