The find familiar text states:
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.
The first sentence of counterspell states:
You attempt to interrupt a creature in the process of casting a spell.
Of course, the familiar is not "casting the spell," because familiars do not themselves have spell slots, but the find familiar text says that it delivers the spell "as if it had cast the spell."
Does this mean that casting a spell through a familiar this way means the familiar can be targeted to counterspell the spell, even though the familiar itself is not using the Cast a Spell action, because it's treated as if it was casting the spell?
Or would counterspell only be able to be cast on the person actually expending a spell slot to use the Cast a Spell action, i.e. the wizard itself, not the familiar?