There are a couple of ways for a creature to lose its dexterity bonus to armor class. I am used to 3rd/4th/5th monster manuals, where dexterity values are given, but in AD&D 2e I can't find those in the monstrous manual. If for example, a monster charges and hence loses its dexterity AC bonus for the rest of the turn, how do I figure out how much worse its armor class gets?
Monsters and PCs don't use the same math for their statistics — monsters don't have Dexterity scores, let alone a bonus for a high score. There isn't even a “hidden” Dexterity bonus to AC included in monsters' final AC numbers that you can reverse-engineer. You can see this in the Halfling entry in the Monstrous Manual/Compendium: even though they are famously nimble and you'd expect them to have at least a default AC 9, without armour a halfling's AC is 10.
To can complicate this if you want and make custom stat blocks for individual monsters who have an AC score one or two points better due to being extra nimble, and then make a note that under circumstances where a PC would lose their Dexterity bonus this creature has a worse AC, but this is extra work that the game absolutely doesn't expect and isn't designed to make you do.
There are a few creatures where their rated AC could be argued to be from being nimble (sylphs' AC 9, for example? maybe?), but again, the game doesn't expect you to make it this complicated and there's no help it offers to figure this out. If you are using a monster that you strongly feel should be easier to hit when it's not able to manœuver freely, follow your intuition and give them a worse AC in those circumstances. Making those decisions is the DM's prerogative in AD&D 2e.
But really, the normal state of the game is that when a monster charges, you just subtract the standard 1 point from its AC.