Dentists or someone who can help!?

My mother has had dentures since she was 17 and she is now 46 they are starting to wear out and one tooth in the denture is breaking. They are supposed to have a lifetime garuntee on them but the dental office she went to when she got them is no longer open. Is she still able to get them replaced for free or does she have to pay for a new full set? She doesn't have health insurance and she cant really afford it. Another question does anyone know how much a top set of dentures runs for?