The use of force, but not deadly force, against a mental incompetent is justified:
(1) if the actor is the incompetent's guardian or someone similarly responsible for the general care and supervision of the incompetent; and
(2) when and to the degree the actor reasonably believes the force is necessary:
(A) to safeguard and promote the incompetent's welfare; or
(B) if the incompetent is in an institution for his care and custody, to maintain discipline in the institution.
Official text of the Texas Penal Code as published by the Texas Legislative Council.