[Lauren]

Nothing you say can ever "make it right" for some that is seriously considering committing suicide. All you can do as a friend is to show them that the effects of them doing this is widespread - let them know you care.
Often, suicide can be a cry for help, they need someone to know that you care for them and losing them would be a terrible thing to happen.

Try not to say "Don't be silly" or "Don't do anything stupid" - because this is a form of invalidation and doesn't help diffuse a situation.

Talking about problems can really help with someone considering this, and let them know you are there for them to talk to whenever they wish, and you will never judge them on anything.