Apparently the mando was a new addition to the hole this year, so I don't think there is a "he's played the course how many times and he still didn't know" argument to be made. Doesn't excuse his not reading the caddy book and noting the mando on his own, as the info was provided to him and it wasn't as if the book and the marking on the tree weren't clear (yes, he's colorblind but there were still flags marking the DZ near the tree to alert him).
Regardless of that particular incident, the mando itself seems poorly executed by the tournament. Whether it was a new addition to the hole or not, at the very least they should have anticipated that there might be problems identifying throws that miss the mando and put a designated spotter under that tree. The more apparent problem to me in that situation is not that Simon should have known about the mando, it's that even if everyone in the group were paying attention to the throw, no one standing at the tee (players, spectators, or tournament officials) was in a position to make a definitive call on whether it made or missed the mando.