Manage the environment, try and minimise the risk by removing things liable to assist someone attempting suicide.
Talk to them and listen, try to get them thinking about other things, gain a better picture of triggers to their depressive thoughts and feelings and then their behaviour, including suicidal tendencies, anticipate triggers and devise ways to co-regulate their emotional state, such as diversions or distractions, until they're prepared to identify themselves their feelings, link them to behaviour, link the behaviour to consequences and become better at emotional self-regulation.
There's a lot you can do besides medicine. Physical fitness, sports, social activities, diet all have consequences in terms of positive or happiness inducing brain and body chemistry. Although if someone is suffering serious depressive illness and suicidal tendencies they could require a medical intervention, some depressive illness is actually symptomatic of other things, I doubt if anyone who isnt qualified could tell the difference between bi-polar disorder and diabetes.