From what you say it sounds like the bank are repossessing his house.
They cannot force him to pay all the shortfall in one go. They can't make him do that, he will work out a repayment plan with them for the shortfall.
In terms of a private sale, while it might be better it would require the consent of the ex. If him and the ex aren't speaking, that could be difficult. At least if he signs it away he's getting a chance to get out of this joint mortgage and get on with the rest of his life, especially before he gets married.