I always just listened, a lot. When the person would pause for me to speak, and they always did at some point, I would. I'd say something similar every time. "I've been there and I know it gets better but right now it sucks. It really sucks. No one can feel exactly what you're feeling but you and the best advice is usually your own. If you need me, I'll always be here. Even if its just to listen, I'm here. Even if we just sit in silence, I'll be with you. You might feel entirely alone but don't you ever forget that you have me by your side, hell or high water, I'm there. I love you."
I don't how much it ever helped but it's what I felt so I said as much. Honesty was the best way to go for me.