Well, I don't know your friend....and I'm just a teenager who thinks too much, but....
One, sit them down and have a good, long talk. Don't let them shrug it off and leave. No matter how they smile, laugh or whatever. Find out why, and figure out how to fix it. Then, help them, not immediately, but slowly, showing them the fun parts of life, and making sure they know that you're there in the hard parts. This is not some super insightful advice. This is just how I'd like to be helped when I'm depressed, and how some people have helped me (almost, but eh).
OH, yes. Lean in for the guilt trip too. Make sure they know how others will suffer if they're gone. That they're not taking their life from themself, but from you, from the people that love them, uh, the people they can help in the future, etc.
Aaaand that's all I can help with. Good luck.