The first thing you <em>should</em> do is to report it to a trusted adult or authority figure whether that be their or your parents, or the school counselor/resource officer.
However, after you do that, you can try to care for them and comfort them as much as you can. It is recommended that you don't give them overbearing care as that can cause them to feel worse about themselves. If they do begin to hurt themselves, tell an adult.
Here is the National Suicide Prevention Lifeline: 1-800-273-8255
The best thing to do is talk to them. Don't go straight to the topic of their suicide or reason for it, but ask around the bush. Make sure they stay happy, not fake happy, but actually happy. Stay by their side. For now keep it to yourself, but you should tell an adult when you enough information on why their so sad. Keep a close eye on them and protect them from any type of person that might want to verbally or physically harm them. Make sure they aren't trying self-harm and make sure they know they're not alone. NEVER yell at them unless it's yelling like play-yelling (joking) or something to make them smile like a proud/motivational speech or something. Make sure that you stay with them through everything.
A friend is a friend and that's what they should do.
