TELL them often. Tell them you don't want them to do it and that they are loved and would be badly missed. You must learn to find your feelings and express them for their sake. They need emotional ties to the world as a final block, the last barricade to death. A tiny thread can be all that holds you in the world when you are severely suicidal.
Single persons are at much greater risk.
My Special Help page might help. Of course the person may not even allow you to help at first or at all Try not to get your own feelings hurt. They are not themselves, they are sick. Even if they don't respond to your help immediately, your patience, perseverence, and strength can save them. Try not to push or force - use reason, then let them alone Let them do it in their own time.