Since having my mother-in-law's teeth cleaned and a capped tooth smoothed down, she won't stop picking her teeth?

My mother-in-law (89) has dementia.  She is also becoming more depressed and deliberately eating less than normal. Other than taking her back to the dentist (planning to do) what else can we do to help her stop picking her teeth and to eat more? The dentist suggested using a water pic. We are and she likes it but is picking her teeth even more no matter what we do.