Were the tenants or the landlord the ones using the utility? Whoever the ones using the utility would be the one responsible "to switch over the change of tenant address". The utilities often ask questions whose answers only the persons setting up the account would know. For examples, Social Security Numbers, dates of birth, bank account number.
Presumably, the tenants were the ones using the utility. Presumably, during those six months of using the utility, the tenants noticed that the bills were not coming. The tenants reasonably should have taken steps to discover why the bills were not coming.
Unless the tenants are disputing that the amount is incorrect, complaining about the total amount being high is useless. Had the tenants been paying the bills as they became due, the usage would still be the same.
Some utilities send bills to both the tenants and landlords. If your utility does, the landlord may have thought that you were also getting copies.
What the tenants can do is call the utility and ask for some sort of payment plan.