From the news of the recent suicide of Leelah Alcorn in Ohio (long story, look it up), there has been a lot of opinion said and written about her and suicide, and many people resurrecting the advice to other how to know and then engage them if you think someone is contemplating and even planning suicide.
Well, here's a rule you should always remember! If you are not a professional therapist who has experience with suicide or a suicide counselor:
Do not think you know someone is suicidial and do not engage them in talking about suicide.
You don't know squat when it comes to suicide so don't make matters worse, and you can and often will, if you engage them to talk about suicide, and force them deeper into their own world. You can and likely will push them the wrong way.
The solution is to find someone who does know and talk to them, and then let them decide what to do, and if necessary, engage the person. And don't suddenly start acting and talking differently around them or to them. They're not stupid, and they're likley to react to you the wrong way.
And do not report them to anyone with legal authority. The last thing they want or need is legal intervention where they lose their rights and face legal hurdles. You could be wrong, and they last thing you want is to make a lifelong enemy.
In short, help them by not helping them, but just be a friend, no different than before, just be there whenever they want to be with you. No more. Just be there.