Something I was told when (actually my wife and daughter were told) when I was suicidal. If you think a friend or family member is suicidal, pick up the phone and call 911. This is a legitimate emergency call. It's a good first step to getting some one into the care of a professional, which is what they need. If the EMT's are worried they can transport the person to the hospital where they can be evaluated, given meds or kept in a controlled environment for a day or so.
It may cause some short term issues, but it's a hell of a lot better than spending the rest of your life wondering what might have happened if made the call.