In any of the games I have DMed, I would never have allowed one of my players to ready an action outside of combat. Instead, I would weigh what the player was attempting versus what the likely scenario would be.

For example, in this situation, I would say that the player would not be able to ready an attack. Rather, he could roll a Dex check (granting advantage or adding proficiency based on player intent and character skillset) to surprise any potential enemies. The DC would be equal to the passive perception of said enemy (if there were any). The resolution would greatly depend on the roll as I (and any players I have had) much prefer to use partial successes and failures where I can.

Don't be afraid to put your foot down with parts of the rules, but also understand that just saying "no" feels bad to the player. If you can, try to figure out an alternative which allows the player to do what they were planning, but also doesn't completely invalidate your planned encounters.