Can Social Security be Garnished for repayment of Student Loans

My husband cosigned for our son's student loan. :( He picked at the repayments and kept getting forbearance until they couldn't do it anymore. We tried to pick up the slack but it was to far delinquent. It was charged off with no warning and we were called today and threatened with a suit. My son is indigent and my husband is on Social Security and VA disability. Our big question is should we be terrified that they will garnish his Social Security or what can he expect. Our credit score has already been ruined and this charge-off can't hurt much more than where we are at now. :confused: