If she is depressed, she need strong support of someone, who can understand every perspective of her thoughts. Spend some time and help her to live routine life, If needed take her to good doctor. Make sure, she need to start to work on exercise, take good enough sleep and get healthy foods. It will be definitely work.
Firstly ask the problem, If you have the solution to their problem, so you try to solve it. Otherwise, take some advice from older persons. If she doesn't want to share the problem with you. So you give suggestions to your friend, just take time and think something good.
First, understand what your friend is going through. Be there to listen. Ask them what you can do and what they find helpful. Try to get them involved in outdoor and social activities more often. Suggest her to try some relaxation yoga.