Bankers call cosigners "Idiots with a pen." I cosigned for my daughters apartment. She now "owes" me almost $60,000. I did it with my eyes open.

I am not a lawyer, but I suspect your son will be paying for an expensive life lesson and without knowing anything of the young lady's finances I doubt he can compel her to pay. If it is really a large amount of money perhaps he can declare bankruptcy. Otherwise he is on the hook.

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Luke 22:36