it's one thing to get out of the cab and talk it through with the guy to challenge him on whether it's been thought through carefully, but physically preventing somebody from jumping is, to me, crossing a line of personal autonomy. i don't think there's any justification for that. a lot of theorists on the topic would disagree with me - mills famously argued that the only justification for the use of force is to prevent harm - but self-harm is, to me, in a different category than pushing somebody out of the way of a moving car, for example.
at the end of the day, we're each in charge of what we do with our bodies. there's no other way to build a truly free society. and that necessarily includes whether we choose to continue living or not.