Someone has to act as arbiter, and by default that's the GM, but when the GM decides, what should they decide?
I have no one answer, but a few principles.
If it doesn't matter much, get it out of the way quickly, and defer any discussion about the rules till later.
If the player had a particular expectation, try not to undermine them. I think this is one of the most important things to try to deal with in the moment.
If the player misunderstood an explanation and tried to jump across a 100" wide chasm not a 10" wide chasm, you may need to clarify some other things, but at a minimum, you probably want to say, "you'll just fall to your death, do you want to do something else?" not "are you sure?" "uh, yeah, why?" "ok, you fall to your death".
That applies whether you have someone who knows what the official rules say and was relying on it. If they've set up a shot that depends on the cover rules working the way the rules say and you've never previously altered, it sucks for them to have that yanked out from under them if you improv something instead. Or whether you have a new player who doesn't know what's covered mechanically or not, and tries to do something dramatic like swinging on a chandelier that in-rules doesn't provide any combat advantage. In both cases, the player shouldn't have a hissy fit, but also in both cases, it's your job to do the best you can in the spur of the moment to allow the player's action or give a good substitute. FWIW, I would allow the first player their interpretation of the rules that once, and if it kills an important NPC, I never rely on an important NPC surviving. And for the second player I'd do something like, "make a dex check, if you succeed, attack with a modest bonus (or choose to knock the enemy back)". That fits the sort of action they wanted.
If it's a one-off, it probably doesn't matter much. If it's going to come up repeatedly (eg. rules for hiding), get past the immediate problem, and then review the situation later. Check what the rules really say. Decide if you'd prefer those, or some modification. Check with the player if they have a sensible request, and if so, consider if it makes sense. Then make a decision, make it clear and stick to it.
If you're not sure which rule to go with? Look for easy to adjudicate (if it doesn't matter, you can always go with what's in the book). Look for fun -- the beginner is right, random stunts should TOTALLY be in lots of combat, and it's a flaw in the rules they're not. Look for ones that avoid breaking a tone you're evoking. Look for which way your players would prefer.
Part of this is just, how to make good rulings in the heat of a moment whichever side you come down on.
Part of it is, where do you draw the line between "what happens because of common sense" and "what happens because what it says in the rules". There's a gulf of people's expectations. Both in terms of tone (is this action adventure where heroes do things humans MIGHT be able to do? Or more like an epic norse legend, where great heroes wrestle sea-serpents?) and in terms of pedantry (do you expect the GM to allow an unconscious villain to have their throat slit? or rely on the weapon rules on how much damage that deals?). There's an amount you can stretch to accommodate different players, but only so far: beyond that, you just have to accept you want to play different things.
It's important to figure out if that's happening or not. You can totally have a tone that has character drama all over the place, *and* swashbuckling *and* fart jokes (see: all of Shakespeare). But if 4/5 players want wall-to-wall drama and one wants fart jokes, it may well not work. And the same in reverse.
Likewise, you can easily have some characters who chose well-optimised powers for their class, and some who chose whatever felt cool, and as long as there's not a big difference in power, it's fine. But if some characters want to hand wave away combat to get to the character interaction, and the other characters want to use the class abilities they just levelled up into, it's a stretch to keep both happy. Sometimes you can, sometimes you can't.
But that's often the underlying dynamic when players react in very different ways, they're focussing on different parts of the adventure, and you want to give both what they want, but avoid what you give one player obviating what the other player wants. Eg. if conversation is always pointless when combat happens, people who want to learn about NPCs are screwed. If you let one character do things because they're cool, but everyone else sticks to the rules, the other players are eclipsed. Can you do both, or not?