Bigger problem. If your mother in law chews betel she as bigger problems then just stained teeth. People who chew betel have a much greater chance of developing oral cancer then than those who don't chew. I would urge you to ave her evaluated by a dentist. In the mean time you can use photoshop to whiten her teeth in family photos.
Ortho phosphoric aci. Superphosphozyme by biotics will whiten teeth. Use a qtip with few drops use on teeth and rinse off using water with little baking soda to remove any acid (or just rinse thoroughly).