All the advice given from professionals to anyone suffering from MH issues is to tell friends and family how you're feeling and to reach out for help.
Um. With mild issues, maybe. When things are moderate - severe it's often a better idea to contact professionals.
It is an absolute myth that people who say they're going to kill themselves don't, and people who don't say it then do. It's bollocks. Please can we stop perpetuating it?Sometimes there are no warning signs. However, a significant proportion of suicide victims have sought support in the days leading up to the attempt. Other preparatory behaviours, such as giving away treasured belongings, are not uncommon either.
If someone says they are going to kill themselves, whether you believe it or not, contact the police and get them to do a welfare check. If it is a manipulative bluff, it takes you out of the situation. If it is a serious mental health problem, it gets the person to a place of safety where they can be assessed by a psychiatrist. Win win.
OP, professionals who work with people in crisis have had years of training and ongoing supervision (professional support within the workplace to help them deal with the stresses/dilemmas of the job). They need to have strong boundaries in place to keep their own mental health strong. A common catchphrase is "You can only do what you can do", and it's really important to remember that your friend's illness is not your fault, that you have done your best to support him, and that it is OK for you to be human and to have limits on what you can achieve. Call the police for him, maybe this will be the time that he can start to make changes.