Open the conversation with, "How would you like to have a suicide on your conscience?" When she wonders WTF you're talking about say, "You know what my job is, don't you? Your interruption could be the difference between the person on the other end deciding to end it or continuing to hang on. Don't do that again."

If she doesn't believe you, or doesn't take what you said seriously, write her off. She isn't worth the stress.

Cheers,
Ben