If you can offer advice without her becoming defensive or taking offense to it, suggest exercising more and taking supplements for B and D vitamins. An alternative way to suggest it, would be to ask if she wants to join you for a walk outside. It would be a great time to talk about life in general as well. Of course, that's only a good suggestion if it is convenient and possible, not if you live separately and/or over a long distance.

Exercise, even something as simple as casually walking for 30 minutes a day, has been shown to improve symptoms for a huge variety of mental and physical health problems. Her issues might be coming from something physical health related that hasn't been caught yet or possibly even discovered yet. A doctor might attribute symptoms to a mental disorder when it could actually be caused by a treatable physical malady, like rickets or scurvy.

A chronic issue many people face today, especially younger people, is a lack of sunlight. The body uses sunlight to create vitamin D. With winter making it difficult to get outdoors, it can be easy for some to become deficient. The sunlight can't come from a window for vitamin D, the glass blocks the necessary UVB rays.

Having been diagnosed with many disorders in my life (OCD, ADHD {actually ADD but that's not a thing anymore}, Bipolar type III, Schizoaffective, antisocial personality disorder and GAD to name a few. I do not believe I have all of those but ADHD and OCD seem accurate enough to me), I can say that walking outside has improved my mental health and overall health tremendously. I'm less angry, have more energy and can focus on things slightly longer than I could before.

Reducing my screen time I think also helps possibly. I have a theory about screens, that since they don't produce the full spectrum of light that the sun does, spending too much time staring at a screen increases the chance of vitamin deficiencies and might cause some of the mental disorders that aren't fully understood (so like, most of them in my opinion) or others that haven't been defined.

I've read that it is recommended to walk outdoors, preferably away from motor vehicle traffic, for about 3 hours a week for the body to create an appropriate amount of vitamin D. I can't quote a source on that, I simply recall reading it and I believe it is true. It might not be true. On that note, this obviously isn't a cure and I am not a doctor or a professional of any kind. I simply think it might help and I truly hope it does.