Communication by Motion

Charge: theft of personal property from the person in an amount not exceeding $150. M.O.: taking a ring from the finger of the victim while she slept in her bed in a nursing home. At trial the victim of the crime was incompetent to testify. The principal witness against D was the victim's roommate, W, an elderly lady who could not speak but who, according to medical testimony, had normal hearing. W could answer the questions put to her only by raising her right knee if the answer to the question were "yes" and by remaining still if the answer were "no." In this manner W testified that she was the roommate of the victim, that she knew D, and that D was the person who came into the room of the victim and the witness late at night and took the ring from the victim's finger.

Defense counsel objected to W's testimony on grounds of competency. What ruling and why?