Erase the carving and restore it if possible. Charge him for any costs. It’s not like the Coliseum is in pristine shape exactly. A good chunk of it is missing and the interior looks like the ruin it is. The sad thing is stuff like this leads security and sightseers being kept at a distance. That’s what happened after Michelangelo’s Pieta suffered a hammer attack by a loon.
This is probably one of the stupidest things I've read here today but cheer up, the day is still young.