First, we need to keep in mind the original question - how he can help his friend - and not turn this into a debate about whether or not suicide is okay.
Second, in reference to the orignial question.... Four years ago I went into a deep depression and was suicidal. Why is unimportant. I didn't tell anyone at the time. I sought help mostly by staying in regular contact with a few friends - enough to remember that there was a world of normalcy around me. I wanted out desperately enouogh that nearly every time I was alone while driving the 20 miles each way to and from town, it was a huge battle not to drive my truck into a tree. Two thoughts kept me from doing it - that I had promised my best friend back in high school that I would not take my own life (I had been suicidal then) and that with my luck, I wouldn't die when I crashed but end up a paraplegic or worse.
Then my best friend died while I was in the middle of the depression. Add that grief on top of everything else I was experiencing. I spent a great deal of time contemplating whether or not her death released me from my promise to her. I was still concerned about not dying in my suicide attempt, more than worried about what it would do to family and friends if I died. I wasn't even that concerned about my daughter. Five weeks after Cale died, a friend here did commit suicide. Sent her husband and 2 daughters still at home off for the afternoon, went out to their barn and loaded a hollow-point bullet into one of their guns. Her husband found her a few hours later.
That didn't snap me out of my depression, but as we grieved Kim and I saw the hurt it caused those who didn't know how much she had been hurting, it did snap me out of the suicidal portion. Soon after Kim died, I told another friend about my promise to Cale and that I wondered if I was still held by that promise. She told me yes, I was. I've been depressed a few times since then, pretty severely almost a year ago, and I've had times where I still want to drive my truck into a tree or just go sit in it with it running and the garage closed up. I'm not sure what exactly holds me here - probably a combination of things - because I REALLY want out sometimes.
So I don't have a good answer for you of what to do to stop her. If someone is firmly set on dying, I think there's not much you can do to stop her. But that she has expressed that desire and is still talking to you is a good sign that she's looking for an alternative. And even without any degree or even training in mental health, you can help her by listening. Listening - maybe without any suggestions sometimes - demonstrates she's worth something. If you're worried about what to say or not say, check with someone familiar with suicide prevention, like one of the hotlines. Professional help may be necessary, and I have no idea if drugs will help or not. If she won't agree to professional counseling, she at least needs to get away from those in her life who wish she was dead, if that's really happening. And if that's not true, that opens up a whole other side of help that she needs, probably more than what you can give her. Be her friend, but not an enabler, and know that if despite what you say to her or try to get her to do she still takes her own life, you did everything you could, and it's not your fault.