Ready action says:
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the cultist steps on the trapdoor, I'll pull the lever that opens it,” and “If the goblin steps next to me, I move away.”
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
A single target loading a crossbow is an easily perceivable circumstance. I would happily rule that you can do this. For a spread out group of targets I would require a perception check to see whether you spot the loading enemy before they get their shot off, but that is just personal choice.
You ignore the loading quality of crossbows with which you are proficient.
Because of the time required to load this weapon, you can fire only one piece of ammunition from it when you use an action, bonus action, or reaction to fire it, regardless of the number of attacks you can normally make.
This doesn't mean you don't need to load the crossbow (it still needs a bolt to fire), only that someone with the feat has become practiced enough that they can load and still get an extra shot off (if they have an extra attack). So we've still got loading the crossbow as a perceivable circumstance as our trigger.
I would rule that a character who already has their weapon trained on the enemy (they've readied their action) can still spot the action and get their shot off. Again this would be harder with a larger group but the given situation is a single target, I wouldn't make a player roll to see if they can release an arrow quicker than the enemy can load, aim and fire their crossbow.
As a DM my thoughts would be: the player has already given up attacking on their round in favour of negotiation - don't punish your players for not jumping straight into combat. This is how murder hobos are born.