It's funny how so many people just assume the old bag couldn't do any damage with a knife. Really? I wonder if it was a loved one, perhaps even your child, who was near this lady if you would just say "oh, she really isn't going to use it, or she can't do any damage..."
But, because it's a police officer, then she should just take the risk that ole granny-grunt isn't a real threat....
Do what the cop says....very simple....