Most interesting story and commentary!
It would seem that your vulnerable daughter has has fallen under the spell of a very skillful manipulator who is possibly a narcissist. Unfortunately, these kinds of "high control" situations never have happy endings. A "Cult of One" can be very difficult to break free from.
Make the trip to discover all you can about the family, the young man who has infatuated your daughter to the point of controlling her and the health of your daughter. If she is to be rescued it must be done before too much time has gone by. Best done by professionals.