A Weak Link in the Chain?

In a Legacy GP match that you are observing, Apollo is at 3 life while playing against Neptune, who is at 5 life. Apollo casts Chain Lightning, tapping one of his two Mountains, targeting Neptune, who has several red mana available. Neptune says “it resolves” and marks herself down to 2. Apollo looks at you, waits a few seconds, then announces he’s casting a second Chain Lightning targeting Neptune. What do you do?

Apollo has committed a CPV for not confirming that Neptune was opting not to pay RR to copy Chain Lightning. Issue him a CPV-warning and back up to the point where Neptune has the option to pay RR to make a copy.

Judges feel free to answer on Judge Apps!