Co-signer loan liability when principal dies?

My GF co-signed on a car loan for a former boyfriend (when they were together). He died leaving a balance of 4000 on the loan. He does have an estate and a grown daughter. What is my GF's liability? Does she have to ume the payments on the loan or does his estate, and if his estate re-negs, is she on the hook? If she has to pay the balance of the loan is the car then hers? This is in the State of New Mexico.