What is the legal remedy for getting treatment for a major daughter who is suffering from mental illness....?

My friend is a naturalized US citizen having emigrated from Sri Lanka.Unfortunately his daughter,aged 22 yrs.,became depressive and is not moving out of the house nor is she studying.she spends her time alone at home in the cellar.she was once a meritorious student.Now she is not willing to take any treatment.As per US law,it appears that her parents can not force her to take treatment.What is the way out?On her own she doesnt take any treatment because of her mental condition.Unless treatment is taken the mental condition will not improve.