There are a number of reasons why a person might want to die. Paul, in Philippians, said "For me to live is Christ, and to die is gain." He goes on to say that he is torn between the desire to depart and be with Christ, and the need to stay in order to encourage the church.
People at times also view death as an escape from the anxieties, struggles, and trials of this life. Pain can make cowards of us, and we might want to die because of that.
It's not a sin to want to die. And while I agree that it might be a sign of someone who needs some professional help, as a casual thought, it doesn't always need to be dealt with via a suicide hotline.
That fact that the person doesn't want to kill themselves or have their life ended is telling. It means that they've identified legitimate reasons for continuing this life. It indicates that they are at least to some extent rational.
I would suggest that if your friend continues to express these feelings for more than a few weeks, it might be adviable to speak to someone about the situation who has a relationship with this person. (A parent, shared minister, school counselor, etc)
Good Luck. Your friend is lucky to have your support.