You and your friends can't assume the responsibility of saving her from suicide or making her life better. You certainly can continue to be her dear friends who offer her understanding and support.
Since you say she is getting worse, you need to share your concern with your school counselor (in confidence of course), a teen suicide crisis hot line/counseling center, a therapist you can trust or your parents. You can't let Nina's potential anger at you for telling someone about her pain cause you not to take some action.
Tell her you will help her find help. Offer to go to counseling with her. But above all, don't just keep doing nothing. This is a terrible bind for you all to be in but the big concern here is not whether you all have a rough time taking action, it's about Nina's life. Sure she could be being overly dramatic about this, using these suicide conversations to be noticed and cared for. But I've learned to ALWAYS take behavior like this seriously!
Write me back to tell me what you did and how it turned out. Let's hope Nina can get the help she needs and deserves.