As a teenager, there are some things about your life that you do not have control of.
You are old enough to make your own decisions about some things. You have the right to decide what you will (or will not) eat. Nobody can "make" you eat anything. If this was a parent trying to force something on you, there might be some room to argue. They do have some authority over a minor. But the daughter of your mom's friend--phooey. Tell her to stick the gluten where the sun doesn't shine. She's a bully and is NOT your friend.