Sure, it's possible that suicide is the best way out for some particular person. But it's just as possible that the decision to kill oneself is an overreaction to some experience or event which the person would get over in due time. Because you don't know, you might go wrong whatever you do.
But there's an important point that breaks this apparent asymmetry: if you err on the side of stopping the suicide, the option of suicide remains available to the person -- s/he can do it later or the next day or the day after. If you err on the side of not stopping the suicide, there will be no second chance. For this reason alone, I think, it makes sense to stop the suicide -- even, if needed, by force (e.g., by restraining the person or by calling the police).
Many of those who are seriously thinking about suicide are conflicted and uncertain. Others go forward with cold determination. I would think that the first group is considerably larger. But quite apart from this, members of this group are far more likely to make us aware of their suicidal thoughts than members of the second group. This provides another reason for trying to stop the suicide: given that this person has in some way drawn your attention to her/his suicidal thoughts and intentions, s/he is probably conflicted and uncertain about them, probably looking for someone who cares and can help her/him find a way to cope with her/his problem. So try to be that person. If you succeed, you may save a person's life and do a wonderful thing also for her/his family and friends. If you fail, you can at least take comfort in the fact that it was not for lack of trying.