I would first suggest you to talk to her about what is troubling her, understand her perspective and start being her friend over time. As soon as you get close to her, suggest her your opinion of depression. If she accepts it, you are good to go. Just in case she doesn't, be her friend and let her get over it by giving her company.
Next Steps
if she is not willing to accept that she has depression, the therapy would be ineffective. just in case you're sure she has depression.
Health Tips
you can also try Bach's remedy, it is supposed to be really helpful