I have been there and it’s a horrible and sometimes hopeless feeling. First of all I would not tell someone who is depressed that it will pass soon and everything will be fine. At the moment someone is depressed, they don’t feel like it will ever be fine or that they will be happy and normal again.
I would offer a shoulder to cry on, tell them I’m there to talk or just listen any time, and try to convince them to seek help from professionals if they are at a really low point and having suicidal thoughts.