Ready Action Dispel Magic vs. Spells with Instataneous effects.

So I have this DM that argues that he can ready action a Dispel Magic against a spell with instantaneous effects because according to the wording of Dispel Magic: Choose one creature, object, or magical effect within range. However, in the spells with instantaneous effect: An instantaneous duration means the spell's magic appears only for a moment and then disappears. And he argues that in magical effects: An effect is magical if it is created by a spell, a magic item, or a phenomenon that a rule labels as magical. Therefore, any casting of instantaneous effect spells can be hindered by a ready action dispel magic.

Does his argument hold merit? Is this true RAW-wise?

Just to clarify, he is ruling that a readied Dispel Magic can be used to stop spells like Fireball?

From the spells description:

Choose one creature, object, or magical effect within range. Any ongoing spell of level 3 or lower on the target ends. For each ongoing spell of level 4 or higher on the target, make an ability check using your spellcasting ability (DC 10 plus that spell’s level). On a successful check, the spell ends.

I would argue that RAW this does not work because the wording says that it stops the effect of an "ongoing spell", which implies a duration rather than an instantaneous effect. That said, he is the DM, which means he has the final say in all things, so he can rule as he sees fit.

This totally doesn't work, for the reasons described above.

Also, there's already a perfectly good spell for that -- Counterspell.

Of course, as the DM, they can set the rules at their table, but I'm not even sure what the purpose of them doing this is. If they want their critters to have counterspell, they can just have counterspell. Also, if they're going to have their casters take the ready action on their turn and do nothing, you can change your actions. (But I assume they're also allowing the casters to ready a spell and also take another action.)

Quote from Ronnieomics >>

Does his argument hold merit? Is this true RAW-wise?

This is covered in the SAC (it was in the 5e SAC too with exactly the same answer).

Can you take a Ready action to ready Dispel Magic to stop another spell from taking effect?

The easiest way to stop a spell is to cast Counterspell on its caster while it’s being cast. If successful, Counterspell interrupts the other spell’s casting, and that spell fails to take effect. Counterspell works against any spell, regardless of a spell’s casting time or duration.

With the Ready action, Dispel Magic can be cast in response to another spell being cast, yet Dispel Magic can’t substitute for Counterspell. The main reason is that Dispel Magic removes a spell that is already on a target, whether that target is a creature, an object, or some other phenomenon. Dispel Magic can’t dispel something in advance. If a spell isn’t already present on a target, Dispel Magic does nothing to that target. The best that a readied Dispel Magic can do is dispel a spell immediately after the spell has been cast to prevent it from having any effect after the action used to cast it. For example, on your turn you could say something like this: “I ready Dispel Magic, and if the high priest casts a spell on anyone, I cast Dispel Magic on the target if the spell takes hold.” If the high priest then cast Hold Person on your companion who fails the save against it, you could unleash your readied Dispel Magic and end Hold Person.

So the answer is no but as @jl8te said, the DM is the one to make the decision. But I also agree that if the DM want them to be able to counterspell just give them counterspell, that is also in his powers.

The idea has been thoroughly debunked already, but I have another point to add. Reactions, unless otherwise stated, always occur after their trigger, so by the time your readied dispel magic was actually cast, the spell would be resolved.

Quote from Ronnieomics >>

So I have this DM that argues that he can ready action a Dispel Magic against a spell with instantaneous effects because according to the wording of Dispel Magic: Choose one creature, object, or magical effect within range. However, in the spells with instantaneous effect: An instantaneous duration means the spell's magic appears only for a moment and then disappears. And he argues that in magical effects: An effect is magical if it is created by a spell, a magic item, or a phenomenon that a rule labels as magical. Therefore, any casting of instantaneous effect spells can be hindered by a ready action dispel magic.

Does his argument hold merit? Is this true RAW-wise?

 You or your DM might find these threads interesting:

- Reactions and triggering actions order of resolution
- Readied Action, Vortex Warp: Does this work?
- Can you ready an action to be triggered after an attack is made but before the damage is dealt?
- Interrupting spellcasting

From those threads, regarding the Ready action (similar to what Maruntoryx explained):

Quote from AntonSirius >>

This wouldn't work, because the attack or casting would be complete before your Readied action would trigger

When the trigger occurs, you can either take your Reaction right after the trigger finishes or ignore the trigger. 

Quote from The_Ace_of_Rogues >>

As a rule, anything that doesn't explicitly say it interacts with a part of the Attack Action- such as Shield, Silvery Barbs, Bardic Inspiration, etc.- can't modify the sequence mid-action. 

Quote from Plaguescarred >>

If you Ready to attack creature as soon as they start casting a spell, i'd rule when the trigger occurs, you can take your Reaction immediately after the spell is cast, not before so.