Tomorrow, my grandmother is having her other knee replaced and is still giving my mother grief about the whole physical therapy thing. I helped my mother out by letting her know that she can talk to her doctors. What my grandmother tells them and what she tells us are two different things and by my mother talking to the doctors, the doctors have a more complete picture even though they can't tell my mother anything. I know that my mom is overwhelmed and nervous.
Would your grandmother be behaving this way if she were thinking clearly? If not, then, I'd try to adjust to the possibility that she just can't and won't. It can frustrating.