Many people have a misconception that as a co-signer, you are not responsible for the debt, or you are less responsible than the primary borrower. Unfortunately, co-signers are just as responsible for the debt as the primary borrower.
Basically, what the lender is saying by requiring a co-signer is this: "We would not loan money to the primary borrower without the co-signer agreeing to pay." In other words, the primary borrower is not sufficiently credit-worthy in the opinion of the lender.
What can a co-signer do if the primary borrower does not pay the debt. The short answer is, get out the checkbook. If the co-signer had the foresight to have a written agreement with the primary borrower that the primary borrower was responsible for the debt, that still does not affect the rights of the lender to make every effort to collect the debt from the co-signer. In that very unlikely event there was such an agreement, collecting is going to be tricky.
First, the co-signer needs to locate the primary borrower. You would be surprised how many times the best of friends, family members, significant others, etc., become separated when the evil spectre of looming debt appears. Then the co-signer needs to try to collect. Obviously, if the primary borrower is not paying the debt, he or she is probably not likely to respond to a polite request from the co-signer to do so. The co-signer can sue the primary borrower, but the co-signer will need to demonstrate to the court that the primary borrower actually owes the co-signer money.
Even if the co-signer can find the primary borrower, and sues the primary borrower, and actually gets a judgment against the primary borrower, there is still the tricky matter of collecting. Getting a judgment does not put money in your pocket. It gives you the legal right to collect. If the primary borrower does not have the money to pay the debt, finding the money to satisfy your judgment can be difficult as well.
And if the primary borrower files bankruptcy, it is game over for the lender and the co-signer getting any money at all from the primary borrower. If the primary borrower files bankruptcy, and the debt is discharged, the co-signer is completely responsible for paying the debt.
The moral of the story: If you agree to be a co-signer for anyone, be sure you are willing and able to pay the debt. That, after all, is what you are agreeing to do.