What should I do my best friend is possibly suicidal?

[D.D.]

You have to take this seriously, whether you stay friends or not. One way to report is to tell the police, the school, report it anon ,
these people in the usa might contact the school and her parents (you could send the parents a message anon, in some way),
if you go to safe2tell.org, in the usa, and give them her name, the info you have , like her yahoo account where you saw it, and the name of her school, the province and all that, even a contact, her parents number , so that they will contact them and let them know, You can do that without giving your name.
there may be other anon places to report, so search them.
Just report it, better than her ending up dead and her parents freaking out.