You request advice on getting people to behave within the rules of a game. One way to get them to see things your way is to use first principles, which for games is:
"A game is defined by its rules."
1. Use Analogies
A. Offer the example of common games like checkers, chess or tennis. They all have reasonably concise and well known rules. As soon as one doesn't adhere to the rules of that game, one can't play it fairly with others.
B. Rules in game like Soccer, and American football. For football, compare NCAA rules versus NFL rules:
- Soccer: you know you can't use your hands, so don't or you aren't playing fair.
- NCAA -- one foot in bounds makes for a reception
- NFL -- both feet in bounds needed for a reception.
You (GM / ref) rule on completed or incomplete passes, hands calls, etc.
The first principle applies likewise to your RPG. The rules create the limits of what the in-bounds and out-of-bounds plays are, and ensure fair play for all involved in the game. Since you are working with differing people, and they may have differing personal visions of what an RPG is for their fun time, *this may take more than one conversation.
C. A non-game analogy you can use is art: the rules define what is in the picture (within the frame) and thus is art. Anything outside the rules is dirt on the museum wall but is not art in the context of that museum (game).
How do you introduce this point to people who already play RPGs using a narrative-first attitude? Communicate to them, but do not be surprised if it takes some interactions in game play to do this.
2. What perfect rule set?
I approach RPG with the point of view of "it never hurts to ask" for two reasons.
a. All rules systems are finite, and the players will eventually find holes and gaps.
b. Sometimes, a weird thing players ask for would be fun, and it is time to make a GM decision. Basing such decision in the rules (if you are consistent) provides both fairness and structure.
When I say "no" I use the rules as the basis for saying "no" and apply the philosophical point: a game is defined by its rules.
a. My original experience with this came 30+ years ago as player. Our first few sessions of Traveler was where I had this point rammed home to the point that I "got it." Some of us (players) were trying to turn the Traveler sessions into something approximating the movie Star Wars, which had very recently come out. It took the GM a few gaming sessions to get us all to embrace the rules of Traveler (and to read them with more interest). He also had to deal with the fact that it wasn't D & D (Pre 1st edition, and our most common game) and it took effort (and our being friends) to better play in this new box.
The lesson stuck.
b. A few years later, during a two year AD&D 1e campaign that began with "Keep on the Borderlands," our DM was very rules and structure based. Of the players, 4 of 6 players had been DM's in other campaigns. Our styles and his weren't the same. It took some time (and frustration, and much patience on his part) to get us to accept his template. Once we did, the game flowed more smoothly and there were fewer arguments mid-game. It was a very memorable campaign. (A few of us still call each other by in game handles, to this day).
4. Will it work for my game?
Effectiveness will vary from GM to GM and player to player. How strong is your relationship with this person? That matters in terms of how the message is received. The original question addresses two people having a different vision of how to deal with the boundaries. That need not be fatal.
5. Supporting your position.
This all leads you back to the requirement for communication between players and GM regarding -- "what are our expectations" -- to bridge the gap. Same Page Tool may help.
If your GM style, and the game, requires all of the players to be very-familiar-to-expert on the rules, and roll with them, it doesn't hurt to point out a few things.
a. We Are In This Together Appeal
Their adhering to the rules makes it more fun for everyone in the group, DM and players alike.
b. Get the Most fun out of our Game Time Appeal
Pace of play improves when everyone knows and adheres to the rules.
OK, Sports analogies again:
- NBA games where fouls run wild, and every other trip down the court
ends in a foul shot.
- NFL games when they refs drop flags like confetti due to false starts
and illegal alignments.
Those games aren't fun to watch, and probably aren't fun to play.
c. Empathy Appeal
GM puts a lot of work into this to make it fun, a little quid pro quo in terms of being more up on the rules is not too much to ask of all players. Your personal relationship with the players influences the effectiveness of this approach.
Not all player-GM pairings are a good fit. This RPG hobby is something we do in our free time. Trying to shoehorn someone into a style that they don't find fun will eventually end participation. That's not necessarily a bad thing, since you have multiple players whom you are serving as GM.