§ 2810 Surety Liability For Principal

• •If someone promises to pay another person's debt, they still have to pay even if the person who borrowed the money can't be forced to pay because of their own personal problems.
• •But if the person who borrowed the money doesn't have to pay for another reason, like the debt is canceled, then the person who promised to pay doesn't have to pay either.
• •If the person who promised to pay knew about a problem with the debt when they agreed to pay, they might still have to pay even if the borrower doesn't.

Your friend asks you to promise their landlord you'll pay the rent if they can't. Later, your friend loses their job and can't pay, but they're also under 18, so the rental agreement isn't valid for them.

Even though your friend isn't legally responsible for the rent because they're under 18, you might still have to pay the landlord because you promised to cover it.