A cosigner’s credit may be improved by cosigning a loan, especially if all loan payments are made on time. Also, if the loan is of a different type than what the cosigner already has, that can improve their credit mix, which improves their credit score. For example, if a cosigner already has a credit card, but cosigns for an auto loan, they now have two types of credit on their credit report: revolving (credit card) and installment (auto loan).
However, a cosigner’s credit score can be negatively affected if loan payments are missed, because the loan is part of their credit history. If at all possible, the cosigner should try to check on the loan and ensure payments are being made on time, because their credit is on the line.