A couple of points to make, one more positive than the other.
First, be aware that many people recover from depression. I was treated for depression in my early twenties, with tri-cyclic antidepressants and counseling. It was a truly dark and heavy cloud, but it did lift, after no small amount of time. Things were fine, in fact, great, for several years.
Then I slipped back into something very familiar. A romantic collapse was, I think, an environmental stimulus that pushed me back into a state very similar to where I’d been before. Whle I didn’t seek counseling or antidepressants, a lot of what it took to beat the previous experience helped me finally effect some serious life changes and get past it. It took about a year and a half.
Been fine since then, and that was well over twenty years ago. Everyone has some hard times, and just plain unhappy times, and I’ve had my share in the intervening decades. But I’ve never since felt myself slipping back into dark world.
So, people do get over it.
The second part of my offering is not as upbeat. It relates to what I might call a syndrome that I experienced, and that I’ve heard of others, including one of my best friends with his (now ex-) wife, experiencing.
My SO of many years became depressed (this was many years after my last experience of depression). Quite a few environmental factors undoubtably contributed to the onset - I won’t go into them. Nevertheless, she started the big sink, and I recognized what was happening.
As her SO, partner, whatever, it became a mighty concern. Of all the people in the world, I was the only one who wanted to intrude on her funk. Nobody else was trying to have a little bit of Christmas in her world; nobody else tried to “make” her celebrate a birthday; nobody else was hinting that she might want to see a therapist.
In short, nobody else cared, and all those other folks were at least leaving her alone.
I loved her and I couldn’t ignore the self-destructive behavior. What that ultimately meant was that I, as her SO, by virtue of not leaving her to destroy herself, became the focus of her trials with the world.
My purpose is to only tell my story as far as it relates to the question at hand. Suffice to say that the ex is doing much better now (years later), and I watched my best friend go through an identical experience with his then wife. As she slipped off, he was the only one who cared enough to disturb her funk. He thus became the focus of her ire.
I’ll note that both my friend and his ex as well as myself and my ex are now, many years later, and with both of the exes well down the road with therapy, close friends.
Just a caveat, Bearflag70, since you asked. The fact that your lady friend has recognized a problem that must be addressed and is in therapy is most encouraging.