I would like to emphasize once again that if the man (and I assumed that, keyword payout) is no longer listed in the land register. And if that is the case, why should he still be involved in the financing? That might possibly hinder new projects for him and poses a fundamental risk.
He doesn't have to. You are absolutely right. However, there is also ethics, even if values are worth less and less for each individual over time, since the I has now generally come before the WE (fundamentally, but also here). Here: because the financing was arranged jointly and due to the breach of contract, which was also 50% caused by him, one could possibly see a moral obligation that he has to his ex-wife, at least to his child. Whether there is a fundamental risk, one would need to know the numbers more precisely. Because if the repayment is very easy for one earning person, the risk is low, but dependent on fairness. And yes, at some point he might also want to get out because the future still has more in store for him. But then he might have to live with the fact that the installments might no longer be feasible for his ex-wife due to the new loan agreement and that the house would have to be sold. Therefore, for me, it always applies to weigh and decide together over time and not ad hoc, mostly individually, mostly not to be put in a rights-box. There is not only one straight path.
4. Solution: Release of liability of the ex. Guarantee by a third person. Then the financing can run for the 20 years, since the bank practically has two debtors.
If that is feasible, then a lot can possibly be arranged. Then everything is good.