Co-signers ares equally liable for the loan, "jointly and severally," which means either of the signers can pay all of the debt, as long as someone pays it. If the lender doesn't want to refinance in just the daughters name (which they won't do if the daughter's credit is not good enough), then probably the mother's only option will be to pay off the loan.
Disclaimer: I'm only licensed in CA. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.