I have never had a cat that chewed plastic, or even heard of this before. It seems like a very bad habit for a kitty cat to have, since plastic is so dangerous (especially the plastic bags) and they can easily choke on it. Even of they are just chewing it into chunks, I am sure that it is not good for their digestion to have plastic bits in the stomach.
Maybe , if you can grow some "cat grass", she would like that and chew it instead. I don't have a cat anymore, but what i do is take wheat berries and plant them in a shallow container, and then the cat or dog can chew on the wheatgrass, which is very healthy for them, and would not be harmful if the cat swallows some of the grass.
