What, exactly, could someone have said/done to maybe prevent a suicide?

Recently, on FaceBook, I saw a post about someone (with whom I was vaguely acquainted) who had sadly taken her own life. The thread was filled with posts from well-meaning acquaintances saying such things as “I wish I had known she was depressed, I could have helped”. “If I had known things were that bad I would have said something.” If only she had sought help…”. This is not an isolated case, I have seen/heard such reactions again and again to news of a suicide.
Why do people assume that the suicidal person didn’t “reach out”? And, more specifically, what could the reached-out-to layman say or do that would actually help somehow?