If somebody has discolored teeth from chewing betel nuts?

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).
Two things. Whitening toothpaste everyday and regular cleanings at your dental office at least twice per year (in addition to decreasing coffee consumption and tobacco use).