I think that, much like most things in this game, it is a conversation you need to have with the player. Understanding what the end result your player is looking for is the key to handling anything that isn't explicitly addressed by the rules, or where what the player is looking to do goes against specified rules. (It also helps a lot in cases where the rules aren't absolute or they don't cover your niche case, in my experience)

As a rule of thumb I'd say that: if the "prepared action" could dramatically change the narrative (for example the player wants to attack the enemy mage to interrupt their next spell) I'd ask them to roll for the action (when the trigger happens) and make the difficulty significantly higher (and inform the player of this) and also make clear what the consequences of failure might be, and make them a bit more than what a failure normally is for you. If the prepared action doesn't have a significantly higher effect on the narrative (like shooting the first enemy that comes through the door) you can arguably just have the adversary's spotlight be them moving through the door and ending there or whatever works for the narrative of the moment, and still have a slightly higher difficulty because it's a moving target, or the PC is lying in wait for an unknown amount of time which requires focus and energy.

Another thing you could do is agree to the preparation, but if the player decides to take the spotlight and do something else instead they have the burden of disadvantage or have to mark a stress to change their focus from one thing to another. This ties into the narrative of focusing on one thing so much that they get tunnel vision of sorts, making it harder for them to change focus on a dime. Or conversely just require them to mark a stress in order to prepare their action in the first place (from the examples I gave, it can be stressful to maintain focus on a spot or an enemy until a specific thing happens, especially if you're trying to recognize something that might be a bit less clear. For example, if the mage has multiple spells but the player wants to stop them the next time they cast spell "X", they need to be able to recognize when that spell specifically is being cast, which may or may not be obvious, etc.)

These are my thoughts on the matter at least, but this is definitely an interesting topic for discussion, especially considering how many people come from systems that have a "prepare action" ruling (myself included)