Can't he do a chargeback on his credit card for the bill and then send the credit card folks a recording of this? Gyms are typically known for making it extremely hard to cancel, like requiring you to snail mail a notice and even then acting as if they never got it.
Chargebacks are no different than simply ceasing to send in payments for a service: It sounds pat, but can haunt you years later when massive penalties and interest appear on your credit report.
The chargeback is a contractual dispute resolution process. The consumer says they shouldn't have to pay for a valid reason. The merchant gets to provide evidence otherwise. A decision is made and both parties have agreed in advance (via the merchant agreement and cardholder agreements) to be bound by that decision. This is one of the consumer protections that make using plastic safer than using cash/checks.