Depression can lasts a long time, and in some cases might even leave long-term mental scars.
Speaking as someone who had bottomed up and left it untreated for years and years until I ended up in hospital with malnutrition, panic attacks and fear of people.
Medical help is needed, but it doesn't mean they need to be left alone & medicines do all the miracles.
Now, what you can do, is as others had said, please stay by their side. Show them that you care. But try not to be too intrusive.
What I did what pretty much disappeared for a long time, and kept in contact only very few people. I used LINE to talk to a few friends. The app doesn't show at what time the users were last online, or online at all. It was what I wanted. Something not invasive. With cute stickers. Your friend may has different needs.
I quit social medias and it did me good. I only started to use them again after a very long time. Very slowly.
Now, what my friends did, was not continuously asking how am I, but for months and years, they kept checking on me. Some show that they're always reachable for me to talk to. Even in middle of the night. (I never tried to talk to them in impossible hours, but knowing that fact made me feel safe.)
Once or twice a week, some would sent me pictures of their dogs. Their meals. Some songs. Film. Random funny things. Idle chit-chat.
People who lived near me called me out for eating together, insisting even if I tried to refuse. It was a good thing, because I neglected eating.
That kept me sane.
Some people with depression would only talk about negative things to the point of irritation. So even friends may feel overwhelmed and tired by listening to them for a long period. It's okay.
It's like being hurt and bleeding. Other people can't feel the pain, but one can try to understand that they're hurt and need help. Don't try to tell them to "get over it" and "It's a phase that will pass". It's like blaming them that they're the core of the problem. It's like telling a bleeding person to jump and run like their pain doesn't exits.
It's okay to not feel forced to be bright-happy-shiny-flowery-person all the time. No need to rush.
Leave space, but stay by their side.
I hope for you and your friend the best.