My question involves the State of: Florida and Michigan

My mother (age 80) was a cosigner or maybe a co-owner on a Capital One credit card with my brother. He had aquired over $14,000 in debt and was unable to pay it off. My mother paid his debt and cancelled the card. Unknown to her, he called Capital One and told them he lost his card and they issued him another card, with a different account number. Without my mother's knowedge, concent, or approval, Capital One placed her on his new card. Earlier this year he died and Capital One is telling my mother she is liable for his debt.

My mother feels she is not responsible for his debt because she cancelled the card and he asked for it to be reinstated and she did not consent to her name being added to the card.

My mother lives in Michigan and my brother lived in her winter home (trailer) in Florida. What should me mother do to resolve this issue?