One of the characters in the party is a Knight with slide 2 on their Melee basic. The general consensus in the group is that when an enemy attacks one of the knight's allies, the knight successfully interrupts (using his battle guardian attack) and slides the enemy out of range for their attack, they lose the action, since it can't be completed.
However today a situation came up where the enemy announced their attack against an ally of the knight, within the knight's aura, and the opportunity action attack from the knight hit, sliding the enemy, but because the enemy had reach, they were still in range to attack somebody, just not the original target, so the DM ruled the enemy could still make the attack it started against the different target.
We just rolled with it, but I'm not sure if that's the right ruling, or even where to look for the rules that would cover this (the opportunity action and immediate interrupt rules don't seem to go into it).