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At eighty, there's no chance you will change his mind, so it's best to not even try. One of the best ways to diffuse the tension when someone is being unreasonable is to say "You may be right....we learned a lot from this situation and won't make the same mistake again." If he keeps at it, just keep saying "you may be right." If you don't argue against him, he'll eventually lose interest (hopefully). He may be afraid that if he lends this money the spending will start again, so that's why adding "we learned a lot..." may help convince him the two of you have no intention of going into debt again once this one is paid off. There's nothing you can do about the debt of the past, but you can offer assurances that it won't happen again...which may be, at the botXXXXX XXXXXne, what he wants to hear.