Not mocking her, EDUCATING HER.  If you are eating so fast you swallow a 2 inch piece of plastic....
I'm going with "chew, don't inhale" and "when I'm eating and I chomp down on something that isn't right, I investigate."

As was previously stated and for the benefit of anyone who has NEVER been to a Subway, the sandwiches are made right in front of you as you watch.  Maybe she asked for plastic?  I didn't know that was an option.