Mother-in-law Let An Account Go Delinquent In Husband’s Name

My husband’s mother opened a Verizon account under his name back in the summer of 2020 and it now has a derogatory status on his credit report. He agreed to opening the account under his name because his mom said she would pay for his monthly phone bill. He claims he doesn’t remember the reason as to why the account needed to be open under his name (he was a senior in college at the time and he loves his mom so I assume he didn’t question her further because he trusted her). Last year in October 2023, he joined my AT&T account and told me he closed his Verizon account (I was aware that the account was in his name because his mom had missed payments and his phone service would be shut off until a minimum was paid). However, apparently he only closed his line and not the account so his mom was still on the Verizon account (she always managed the account, he didn’t get the bills or have/try to have actual access to the account). This summer, my husband got a call from a debt collector about the account. It has a balance of about ~$2,200. My husband never actually had access to the account because his mom set everything up, but he still signed for it. What legal action, if any, can we take so this isn’t stuck on his credit report and we are responsible for paying the balance?