You conveniently left out the fact that he was making her wear a mask, say "kill the pigs!" and hold a gun. She was senile so no she isn't able to make her own decisions because her mind is deteriorated. That's probably why she lives at home with her family and not by herself but she would probably be better off on her own after this.
I have a grandfather who is worse off than her but if I were to do that in the hospital the staff would call the Police on me in a heart beat and rightfully so, you should respect your elders and cherish the time you have with them not humiliate them for cheap laughs.