first contact the charging company and see if they can reverse the charge.
The charging company doesn't know what amount he authorized and I'm sure couldn't care less. He owes them $51 and that's what they charged him and, if the storage rental contracts I've seen are the norm, he already agreed to pay that $10 when he signed up. If they were to reverse the charges, he would probably then owe $10 for each day he hadn't paid or some other fees.
If you owe it, just pay it. You might ask your CC what their policies are so you are clear on exactly how they handle such transactions for future reference.