Vickie writes, in response to my last post about what not to say when a friend tells you about something she thinks is good news, "I would LOVE to see you write a posting about WHAT TO SAY when you are the friend." I think that's an excellent point -- I spent my whole post talking about what not to say and never gave any suggestions for how to be supportive.
Probably the number-one thing to do would be to listen. Sure, all of us can go on and on about something we're excited about, to the point of being boring. For the first two or three iterations, I think the friendly thing to do is to listen anyway. Try to see what is so exciting about this news.
If she's happy, be happy for her. As long as the exciting thing is not obviously life-threatening ("I met this great guy on the Internet last night and he's taking me to his cabin in the woods this weekend!") it's probably best to try to have faith that she's smart enough to figure out the details. "So you're quitting your job to write a novel? That sounds exciting! What's it going to be about?" The nice thing about it being her life and not yours is that the consequences are also hers and not yours. Let someone else be the wet blanket. Like I said, "no one will love you for this, even if you're right. Actually, especially if you're right." I think the nicest thing someone could do for me in the situation where I did do something dumb, and crashed and burned, would be to say, "I always thought you could do it too. Sometimes things just don't work out." Not, "I always thought it was a stupid idea."
And what if you're jealous of your friend's good news? Admit it to yourself. Don't try to pretend that you're really worried that her diet is bad for her health, or that her new job is going to be too much for her if it's really that you're green with envy. Once you admit it to yourself, you probably won't need to passive-aggressively make her feel bad for reminding you how much your life sucks.
I don't know, though. It's one thing to act as if you believe in your friend and are always happy for her when she's happy, and there for her when she's sad, and that you trust her to figure out her own problems. It's another thing to really be that way. The best friend in the world would actually be those things. I have had a few, and felt really lucky. I can't say that I've always been that person myself, but I am trying.