To be suicidal, you have to be planning your own death, or thinking of ways in which to kill oneself, though the plan doesn't have to be carried out. It is the desire to take your own life, essentially.
Immediate course of action is to remove them from the situation, if caught, and seek some form of therapy and a course of antidepressants. If they don't seem to work, and the person is still attempting, hospitalisation may be the only route.