Has her family considered getting her a Christian therapist? Youth leaders and parents can only go as far as they are able, which they may have already. Perhaps she needs a higher level of counseling then what's currently available to her.
Also, at the end of the day, the friend who stands by her, prays with her, and simply loves her is the greatest support. No magic phrase or enlightening word of advice will end her situation immediately. It's a long-term struggle that she may have for the next several months.
Something you should try next time you're with her and the voices are loud. Instead of telling her to pray herself, drop down with her and pray aloud over her. Pray for the voices to silence, for your friend to feel His presence and peace, and for her to know that her life is valuable in His eyes.
If that's awkward, I completely understand. However, when all words fail, the greatest gift you can give to her is to be with her, cry with her, fear with her, and let her know through your actions that she's never been alone in this struggle.
Keep it up, loved one. We really want to read this.
Do you follow her on Pinterest/Instagram? You could always talk to her about the things she's looking into and address the thoughtless friends who post death jokes (once again, awkward, but necessary).
One thing you could tell them. God will hold us accountable for every foolish word we speak on the day of judgement. Every death joke, every lie, every idiotic remark will be held to us... yet another reason Jesus's taking all of our sin to the cross is so remarkable. He died for every stupid thing we'll ever say about ourselves or to other people. It's truly incredible!
However, it's good to keep in mind, God can hear you. He listens to your murder jokes and thoughtless speech. Is that what you want to be known for?
Now I'm not saying you should tell them that word for word. However, seems as though your friends ought to meditate on Psalm 19:14.
"May the words of my mouth, and the meditation of my heart, be acceptable in Your sight, O Lord, my rock, and my redeemer."