Yes, it’s a somewhat common fear. Some options to help your grandmother: the dentist can use a rubber dam or isolite system to help minimize the water in the mouth. If the anxiety borders fear, she may require sedation. The body has natural reflexes, which will obviously prevent choking. This is more a psychological issue, so reassurance is important. Have a consult with the dentist and hopefully he can create trust, showing care and empathy.
I wish your grandmother well. Hope this helps.