I am myself going through depression ....one thing you should understand first of all that "cutting her hand" or "getting extreme sad" are all symptoms of depression . She wont be able to understand it till she get proper medication . Go To general medicine doctor [1]..they will refer her to neurologist or profesional psychotherapist if he feels needed. They will be able to understand whats going wrong with her and they wll guide. Next pointers parents must be extra vigilant and try to avoid her being alone and remove door locks [ only in cases where person tries to cut hand or similar act] . Do test her vitamin D lvls or ask doctor to check if she gt vit D deficiency [ symtoms of vit D deficiency according to what i have understood during my treatment : cant walk well on stairs...bone feels like they wil break...and tendency of falling on street due to small bumps ] Lastly really take care of her and when ppl get depressed sometimes few take certain suicidal steps which they themselves laugh cause disease makes them think that way, in reality they actually dont to die . Most importantly this health issue requires serious family support from parents as well as friends cause depressed ones always get stubborn and say in their mind " i am alone ..no one loves me " . [2]
[1] You can also visit pshyologist directly cause in this case things have got realy messy.
[2] See if she can meditate a bit everyday ... allow her to chat as much as possible but avoid social networking sites like insta where users only show their "best moments" in life .
Best of luck mate... do dm if you need any further suggestion. I dont want anyone to ever take their life just cause the disease makes them feel they need to die asap.