Is this a bad idea? It would be odd for the protagonist to suddenly dwell upon those past events himself,
Yes, I think this is a bad idea. Non-POV characters that come and go are foils for the MC (POV character) somebody in their world to interact with, to reveal new information, make a social setting plausible, cause the MC to do something, provide a bit of help.
ANY POV character should be central to the story, not a convenient side show. Even if you could make it clear the new POV character is not going to reappear, this looks "too convenient" to the reader. For one, the new POV character is a complete stranger to the reader, disconnected from everything that has gone before: A stranger knocks on the door, explains something crucial, and walks away forever. It doesn't fit; it's implausible and just like a dues ex machina.
1) I would devise a scene in which a walk-on character (not POV character) more naturally reminds the MC of something that causes the MC to reflect on these incidents, so it does make sense.
2) Or make the incident bigger, involving more people perhaps, or make it more newsworthy, so reminders occur more naturally. Something new about the incident is in the news. A walk-on contacts the MC to find out if they know something -- No, but it is an excuse to talk about the incident.
3) Or my favorite: Don't explain. If the MC has no reason to think about how their story and the other stories are tied together, and the other POV characters have no reason to think about that, then how they met does not matter. By "matter" I mean it does not really make any difference in the plot or how they act; because if it did make a difference in some decision or action or emotion, that is the opportunity for the author to explain why a character is doing something based on how they met, or their past interactions, etc.
Otherwise, many stories are written about married couples without explaining how they met and got married, say a story about them dealing with the illness of their child.
An author should not include incidents (or go to these kinds of lengths to include incidents) that make no real difference in how the story in this book will progress. If they do matter, rethink the ACT I (first 30% of the story) scenes, minor problems and issues and walk-on characters, where the MC is introduced, to allow reflection or reminiscing on the memories of these past relevant actions.
You don't have to be blatant about this: Something causes the MC to be stumbling about in the dark in their apartment. The power went out, it is overcast and pitch black, they are searching for the flashlight on the mantle and accidentally knock something off with a sound of shattered glass. Great. The flashlight is found, the broken picture is of the MC and three friends at summer camp, a favorite. Fortunately the photo itself is undamaged. While the MC, by flashlight, is picking up sharp shards of glass, they are thinking on the adventures of that summer, Jill's stupid idea to build a secret raft ...
In ACT I, such "minor problems" serve to introduce our MC and provide reasons for character building, and these minor problems can be plausible, solved and dismissed without the reader expecting them to be truly related to the plot. Such minor problems are an excuse to build character, either in how they deal with problems (Are they inventive? Had they planned ahead?) or as an excuse to do something unusual; like take a different route to work or do something out of the ordinary that builds their character, call a friend, look in the attic for something, stop at a shop they haven't been in before and happen to meet somebody new, etc.
Invent some reason for your MC to reminisce or meet somebody from back in the day. Heck, they get an online friend request from somebody back when.
I would not introduce a stranger that walks on stage, dumps some info, and walks off never to be seen again.