hawkeyefan
Legend
except it’s not because rpgs have these things called rules, ya see.
but hey, thanks for contributing nothing to the discussion
It’s all a matter of play preference. What you consider “cheating” isn’t considered that by others. So telling people that they allow cheating in their game is kind of rude. And also is factually incorrect as I don’t believe that you’ll find anything in 5e that says this is cheating.
Having said that, in looking at the two approaches and how they’d apply to the scenario in the OP....
In one, the DM simply lets it stand. They ask the player how the PC knows this, and then incorporates this into play. Play then proceeds with the party being on guard around this NPC.
In another, the DM tells the players to disregard what they know. Their characters do not know. Perhaps this is determined by checks of some sort, but if those checks fail, then NO ONE KNOWS THE THING WE ALL KNOW. Play proceeds with everyone pretending to not know she’s a possible enemy with an ulterior motive. And everyone must act surprised if she reveals her true colors.
They’re both approaches to play, and different groups will have different preferences. For me, the second option sounds distracting and frustrating and for very little, if any, benefit. Any kind of twist with a NPC revealing their true colors loses any impact if the players are already aware of the twist. So it just seems better to allow the knowledge to happen rather than bog down play out of some misguided attempt to avoid “cheating”.