Does Zone fitness have a right to charge me?

Is it normal to be charged for another person's account if you only signed for them on their contract?

A friend used my account for the first few months. Then he opened his own and provided it to the branch and they started deducting from him. Once he stopped paying the gym fees the branch started adding the payment to my name. Now its almost R4000 overdue and they have handed me over to collectors. It is now affecting my credit record?

I have a membership as well but I have been paying it monthly without missing a month.