Wrong address

After we were notified of possible legal action by a third party who has nothing to do with us, Pathcare wants to instituted legal action on an an outstanding account which we never received. The account was send to my mother's ex-husband's postal address and not our own postal address. Upon phoning the Pathcare accounts department, they told us the doctor said that the account has to be sent to the ex-husband's postal address. We ourselves don't even know what's his postal address and the doctor doesn't know also. How in the world they got this address is beyond me if we ourselves don't even know what it is. The laugh of it all is that another Pathcare account was sent to our own postal address. What I can't understand is how come they didn't use our address with the first account. Get all your ducks in a row Pathcare and don't use other people's postal addresses and complain when people don't pay the account.