Well, for starters, it is good your friend can at least put what she feels somewhere, comunicate it...
Beleive me, pretending to be happy all the time, that everything's fine and then suffering in silence and alone is much harder and painful, I know theres people who do that... the fact that she is able to comunicate it and talk it out is good for her! Try to keep her that way and hopefully she'll be able to at least fight it out and maybe some day get past it.
I'm not taking or trying to take away the importance of your friend's message, because obviously it a very serious issue, but it seems to me that in this day and age 80% of the population is "depressed" and I am beginning to feel as if they don't really understand what depression is. Instead of saying "I am sad" they say "I am depressed" and to me it just seems a tad bit ridiculous. Either many people are mistaken about believing the two words and interchangeable or it is because with the introduction of phones and social media those people have a form of communicating their feelings, and more people have a form of communicating their feelings. Maybe I am the one that is mistaken but I'm just stating what the situation seems to be to me.
I think I get what you mean.. people nowadays seem to confuse those two concepts and place them as synonims when they are not... Everybody gets sad, its something that just happens for one reason or another, and then just ends. Depression on the other hand is a much complex thing... the main difference people dont seem to get between these two is that, sadness is a feeling... while depression is more of a "condition" a person can be at for a certain period of time. Depression includes sadness, and I think that from where people get the concepts mixed up.
Theres a lot more to mention when it comes to this topic but of course it would get too long so Ill end it up here.