The scenario is as follows.
They rolled an 11 or higher and thus will only complete it on their turn.
When your turn comes up, before the enemy caster has completed their spell, can you prevent it from completing by casting dispel magic?
Dispel magic states the following
Choose any creature, object, or magical effect within range. Any spell of 3rd level of lower on the target ends.
As the spell has not completed, is the process of casting fireball considered a spell, or is it not a valid target for dispel magic?