The term "Depression" has entered the common language, and lost much of it's original meaning. We can be talking about down in the dumps and don't feel like doing anything or we can be talking about clinical depression, a serious mental-health problem. If it is to the point that it is interfering with her relationships, I think the best favor you can do her is to steer toward getting the condition assessed and treated. If it's mild to moderate, St. John's Wort might be an option. The research on the benefits of it for severe depression has been very mixed, so I am still recommending a referral to a physician or therapist for severe depression. Although ti would be an experimental treatment for depression, there is a technique called Emotional Freedom Technique that seems to have some promise and many people are reporting benefits from it.
A very good place to start educating yourself about depression is "The Feeling Good Handbook" by David Burns, MD. He very frankly establishes the difference what you can deal with at home and what needs professional attention. It is probably the best book for lay people on depression in existence. (I'm not affiliated with Dr. Burns, except that I use his materials in my counseling practice.) I would recommend that you start with getting that book, and leave it on the coffee table.
