Things I've said to my dp which he tells me have helped -
It's ok to cry, and I'm not judging you for it, I'm just going to sit here with you and hug you while you do.
I'm sorry you're going through this, it's shit and you don't deserve to feel like this.
You don't need to talk about it, but I'm here if you want to talk.
I'm not going anywhere and your depression isn't going to scare me away or make me love you any less (and then quietly and consistently following through on that)
It's fine if you can't make that decision right now, we'll come back to it/I'll deal with it (again, following through is important)
It's shit. It really is. But not every day will be like today. In the meantime let's get through today together.
I love you, exactly the way you are, I won't try and 'fix' you but I will be here to support you in whatever you want to do to make yourself feel better, or however you need to cope.
On really bad days he tells me that practical stuff helps, like giving him small simple tasks to do, take a shower, make a cup of tea, make us some lunch together. Having some purpose can help, but needs to be balanced with anxiety.
Sometimes sitting him down and asking him outright to describe what's happening inside his head helps, not looking for a reason for the current bout of depression but just describing how it's playing out in his head. So I say things like, how are you feeling, what are you struggling with, tell me what's happening in there. A lot of the time there isn't a specific reason or cause for a bad day, but if I can understand what's going on in his head, I can help accordingly, and it seems to help him too, to describe what's happening without necessarily looking for a cause or solution.
It affects everyone differently obviously, but I think being consistent, kind, and available helps. Depression can be very isolating, and having someone who will just keep showing up, reaching out with love and affection and no judgment can make a world of difference I think.
It sounds like you're a good friend, I wish my dp had more people like you in his life.