lindsey, i don't have a ton of experience in this area, but i went through some depression and suicidal thoughts when i was a teenager, as well as studying psych in college. i just want to say that i really think you should absolutely forget about the privacy issue. this isn't something to fool around with. if you really know and believe that it's not a joke, and she's serious about this, please don't play around. this is her life you're talking about. which is worse? her committing suicide or her friend being upset with you? think about it that way. if she's serious, she needs help, and she needs it NOW.
i'm not saying that you shouldn't take her to coffee and try to talk to her, but if she's at the point where she wants to do something like this, there's not going to be much you can say to stop her. i just wouldn't want you to not take action, simply because you're afraid to offend someone. it doesn't matter in this situation.
i hope that didn't come across too harshly - i would just hate for this to have a terrible outcome. i'm praying for you and the situation! (((hugs)))