Share information when you have it. Maximize peoples' opportunities to hold informed beliefs.
Surely one must weigh the consequences of changing another’s mind. If there are extreme detrimental effects, perhaps it is not proper to do so.
If you believe you can change an adult's mind in a normal situation (for example, we are not including someone isolated through prison, cults, etc. or of diminished mental capacity) then you're claiming the ability to change their mind to their detriment and their inability to see the impact of that change.
In this situation, you have done something to them like the Pied Piper, leading them away from community...except that the pied piper was only able to lead children.
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.
I'm saying you bear no responsibility for the beliefs another develops based on information you offer, provided that said information isn't intended to manipulate or mislead. It is not a scalable perspective no matter the degree. Artists, for instance, can't worry about the mental state of every member of there audience, even to a tiny degree. This is in opposition to the parent's point.