I will be honest - but this is just my opoinion, and may only fit for me.
The BEST thing my friends can do when I am low, is just be my friend. Not to talk to me about how I feel, or why, but to pick me up and go shopping, or out for a meal, or anything, except put more pressure on me. In those low times, I just want to feel 'normal', therefore I need my friends to do 'normal' things with me.
(This is made easier for me, as only a very small handful of my friends even know I have depression.)
I acknowledge that my friends can't fix my depression, or make it go away, it is for me to manage as best as I can.
That being said though, can you offer to take her to the Dr and you go as support. The earlier a diagnoses is made, the quicker a solution can be found.
And welcome to the boards, you sound like a very good friend that has gone to this effort for your friend. Don't tell your friend how hard it is for you to see her like this, this will likely add to her guilt, and perhaps she will hide it from you in the future