Severe depression

is physically as well as mentally draining. I really wish people who have never been clinically depressed themselves could understand this.
I am not sure I understand it myself. I don't want to be that depressed myself,  of course I don't. I do want to understand though - at least sufficiently well to know what to do.
A friend of mine, someone of whom I am very fond, is going through a period of severe depression right now. It's hard to watch - but it is much, much harder for her.
I've hugged her and she has responded. She knows I am there for her if she needs me - really knows it. No, it isn't empty words. I would sit there quietly and listen if she wanted to talk - but she can't talk. That's the hard part. She just can't do it. 
She just can't do it. It doesn't matter how much she wants to talk the words won't come. Talking  isn't going to help right now. She knows she is loved and supported but it isn't, for now, making her feel any better. 
And I simply didn't know what to do yesterday. She was coming out of a room full of people yesterday. She had made herself go to a committee meeting. Nobody had said anything wrong or done anything wrong but, she told me, it had all got too much. It was too noisy. She needed quiet. 
Her husband was coming to get her. He's been incredibly supportive but it is hard on him too. 
I had not yet gone into the meeting. I asked her if she wanted me to just stay with her until her husband came. She hesitated and then said, "No. I just want to be quiet."
I hugged her silently and left it at that. But I am still wondering whether perhaps I should not have asked, whether I should just have said, "I'll wait with you. I won't say anything. I'll just be here."
The meeting could have proceeded without me. If the others had not understood then too bad. Some of them would have understood and would have backed me to the hilt if I had not gone in for that reason.
I didn't really want to be there myself but I had to deliver some "housekeeping" sort of messages. It was someone else's birthday. There was cake for afternoon tea. I don't eat cake very often - and never there. People sang "Happy Birthday" to the birthday person. I was conscious of my friend, surely by now collected? I couldn't imagine anything worse about that than hearing people inside singing while she was out feeling as if the sun was never going to shine again.
There was something else happening after the cake and cups of tea but I really didn't need to be there. I just couldn't face it. I left. 
No, I am not depressed. Like everyone else there are times when I feel "down". Right now it is hard to watch my beloved Senior Cat struggling but I know that this is not the sort of depression my friend feels.
I just wish I could help.