I’m no attorney, but I think an attorney has a legal as well as an ethical obligation to protect a client who is legally incompetent either to stand trial to begin with, to allow such a client to make statements, pleas or motions that may affect the case or to be sentenced without due consideration of the client’s diminished capacity.
Of course, theoretically the client could dismiss the attorney from representing him/her before the court rules on the client’s competency.