Exploding Kittens Nope Cards
Anyone can play a Nope card at any time (except for an Exploding Kitten or Defuse Card), from the rules:

Stop any action except for an Exploding Kitten or a Defuse Card. Imagine that any card beneath a Nope Card never existed.
You can also play a Nope on another Nope to negate it and create a Yup, and so on.
You can play a Nope Card at any time before an action has begun, even if it’s not your turn. Any cards that have been noped are lost. Leave them in the Discard Pile. You can even play a Nope on a SPECIAL COMBO

So yes, if player C wants to block an attack from player A on player B, they can.
See also how the attack rules explicitly use victim and someone:

(If the victim of an ATTACK CARD also plays an ATTACK CARD, their turns are immediately over, and the next player must take two turns.)
If you play an ATTACK CARD and someone plays a NOPE on top of it, this means that you still have to draw a card and the next player does NOT have to take two turns.