Honest sender, honest receiver: zaps cannot be faked

Honest sender, dishonest receiver: could, but validation on sender side should prevent it

Dishonest sender, honest receiver: cannot be faked

Dishonest sender, dishonest receiver (working together): can be faked, no LN tx even has to take place

Dishonest sender (sockpuppet of receiver), dishonest receiver: can be faked, no LN tx even has to take place

---

Btw, a 3rd party (any nostr account who is neither payer nor receiver in this zap) cannot tell if the zap was authentic or faked.