@AC
Someone illegally used his telephone service to make calls. He may well have had poor security but that doesn't make it his fault or even ok to do something which amounts to stealing off him. If i leave my house unlocked , taking things from it is still stealing. I assume you wouln't complain if a locksmith open your house and took all your stuff, after all it's your fault for having lax security on your front door.
Saying he has to pay this bill is the same as saying if you get your creidt card skimmed your liable for the purchases made on it. Yes someone illegally used your card, but bad luck shouldn't have been lax enough to let someone steal your details.