Suppose a player forgot the correct "book move" in a tournament. He/She looked at another game played by strong players in the same tournament and happened to find the same opening position. So he/she copies the strong players move in their own game.
Is this considered cheating? If so, should the arbiter stop players from spectating other players' games?
When this has happened in high level tournaments being done by top level players the arbiters' decision has always been "No" but it has been a fudge decision where the arbiter decides that the copying wasn't deliberate.
If there is clear evidence the copying was deliberate then the answer would be "Yes" but proving intention is very difficult and arbiters are reluctant to go there.
The precedent set does make it difficult for arbiters in lower level competitions to judge differently unless there was some additional evidence (hard to think what) that made it more obvious than it already was.
The latest well known example is from the 2019 Isle of Man Grand Swiss which had a place in the Candidates as a prize. This was covered in detail in this answer. Afterwards when the Chief Arbiter Alex Holowczak was interviewed by chess.com's Danny King he basically fudged the issue:
Holowczak: "I saw Yangyi look quite nervous when he looked over to Dreev who had made the move a minute or two before. So, I thought in the interests of everybody feeling comfortable with the game that it was better to move it into the second hall. Then nobody can accuse anybody of anything"
King: "Is it an offence to copy somebody else's moves?"
Holowczak: "N'Ermmm ... I suppose it must be if we think they have copied their moves, but we don't think that. We're not moving them because we think there has been an offence. What we're trying to prevent is any accusation that there might have been an offence"
