You most likely have to pay:1) If the practice were a corporation or LLC of some type and that corporation or llc still exists (they do not automatically cease when an owner passes away), you still owe the practice.2) Even if the practice were not an LLC or corporation, any debr owed to the orthodontist personally becomes a debt owed to his estate; the estate could sue you for the money.3) If the assistant did work for you after the orthodontist passed away, you would clearly owe for the work she did...that work, done after the orthodontist died by a still living person, has nothing to do with his death.