Picture this situation. Myself and a friend are a pair of casters, both with familiars. My friend's familiar has just gotten in position next to an enemy we're fighting, and I know he intends to use his upcoming action to have the familiar deliver a touch range attack spell (such as Shocking Grasp), per Find Familiar...
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
My familiar's turn comes up, but I won't need its help when it's my turn for whatever reason. Instead, I want it to use the Help action to help my friend's spell land, so I order it to move next to...
...who? The caster, or the familiar?