It's hard for me to tell what exactly you're concerned about. He didn't injure or probably even bother the crabs. The Sharpie really will disappear within a few days, so even if some future nature-lover stumbles upon the island later, his experience won't be "ruined" by numbered crabs (I would find it charming if I found such a thing!) So why are you concerned at all?
I'm concerned by the fact that he allows himself to go to such a place without applying the precept "Leave no trace". In my opinion if you want to preserve wild areas like this one, it is very important to respect it no matter what.
You would find it charming but I would find it sad. We don't know who else might visit this area and by what frequency, just by respect for other people who may not be pleased by such a gesture and because he doesn't own the place he shouln'd have done that. It really seems just like a basic outdoor camping rule to me.
> I'm concerned by the fact that he allows himself to go to such a place without applying the precept "Leave no trace".
You can't "leave no trace". The animals that noticed you behave differently now. Your footprints diverted an insect. (And, if you were riding a horse, the water that pooled in the hoof prints kept a Texas Ranger alive.) Your "waste" poisoned one thing and provided nutrients to another.
You can argue that these things don't count, but the line isn't as bright as your aesthetic revulsion requires.
I can't speak for cake, but I suspect it's as much an aesthetic thing as anything. Example from art: if you were insistent on viewing the Mona Lisa from a cubist aesthetic, it would be a crummy painting. It's not a moral assertion as such, though.
