Hello. It must be very hard for you to watch your friend like this.
Depression can be caused by many things, it would be interesting for me to know whether your friend has had some sort of extra pressure or added stress to her life in recent times; depression can also be caused from a past trauma or can even be carried in the genes - this is why it would be important for your friend to first see her GP to be diagnosed.
As a therapist myself the other thing I would ask you to do for your friend is allow her, her down time - sometimes depression can be a process people will naturally go into and come out of - it can be the body and minds way of proctecting itself or even having a break from life.
One question i always ask my clients is "What do you need?" it is amazing what people will come out with when asked this question - give it a try and ask your friend what she needs.
Give your friend time and don't push her too much to start with - your gut instinct will tell you if you need to intervene with more authority.
Alison