how would you explain to a person with a dislocated shoulder that the shoulder is likely to become more easily?

This is very well documented.

If you read "Rockwood & Green" Orthopedic books, they suggest that a history of shoulder dislocations in a person under the age of 40 will result in nearly a 100% incidence of recurrent dislocations, without corrective surgery.

That includes those who go through adequate physiotherapy and treatment.