I imagine the line is different for different people, but it could very well impact more than just their life. Say, for example, you have a colleague who is convinced his wife is faithful, but by coincidence you saw her at a bar with a gentleman and quite clearly it was romantic in nature. Do you mention it? When he doesn't believe you, do you try to convince him? What if he is short tempered and known to have a violent side? To be clear, I am not advocating any particular course of action here, just acknowledging there are situations where it is tricky.
The majority of the parent comment is focused on situations where you look to change someone's thinking in an expansive way. New ideas, new philosophies, new ways of thinking...
We could push the boundaries of this and say I am trying to convince a man of something that may or may not be true for his own good.
In the hypothetical you allude to: with just one observation, no matter how compellingly I've stated and restated it, his actions are his own. Otherwise you're suggesting that I told a man I saw his wife at the bar with another man and his actions are attributable to me. At that point how does one even interact in society? Frankly, anyone might be a powder keg given the day.
If we go back to some concerted effort to prove my observation leads to my conclusion which I'm pushing him to believe, I think we've exited "gentler, better" to straight up malicious.
