I was trying to do the same when I was at your age but, thank God, it's really hard to do.
And know one thing, that you will NOT stop to exist after the death of this temporal body. You will continue to exist, but if you leave your body before the time, you will stay in the in-between condition like a ghost. You will be still covered by your subtle body, mind, but there will not be any gross physical body since you've killed it.
Tell her friend, that
Better not do it. Suicide is not a solution.
Whatever you do, you WILL continue to exist. We just change our bodies. We will continue to exist after our bodies die.
Life is so short, time goes by so quickly, why make it even shorter. Everyone who has born IS to die. Everyone who has died IS to born again. There's no need to interfere in the nature. At the certain time all of us have to leave our bodies and get another ones.
We are not our body. We are IN the body. We're the drivers inside of the cars - our bodies, so to speak. When a car crashes, the driver continue to exist.
So, suicide will produce only more problems and suffering.
It's better not to leave your friend alone! Try to find out what's the reason of her depression. It can'be either a broken heart or bullying in the school.. take a good care of her!! otherwise it can be too late!!! if it's really serious, maybe it makes sense to tell about it to her parents.