Talk to Psychiatry on Psychological Disorder

my mother is a psychological patient for about 20 years, this psychological problem has affected her to an extent that she don't eat anything except a small diet in liquid form,this issue made her very weak and at this age of 67 ,she is suffering from tuberculosis and being treated since 8 months,her tuberculosis test reports are normal but she still dont chew and eat anything...actual issue is a fear inside herself that she think that if i eat anything then this could be harmful to her......what should I do? plz help