Nutrition, sleep, screen time, activity. Depression absolutely has two components. One is environmental. That’s the easy part to try to address by changing things to make her environment better for her. If she has bad sleep habits, isn’t eating well, isn’t active or is on screens too much then those are areas to try and target.
The second area is much harder to address because it’s genetic. If someone has bad depression and it’s just how they are wired, it just takes purposefully doing things every day to try and be a little bit better, but will likely be an area of struggle to various extents throughout much of her life.
Look into positive psychology for other areas that are within your control and try to make those types of activities a normal part of her daily life.