Consider dropping it altogether if it's not a big deal. This about learning to prioritize - I had someone comment on my site thinking mistakenly that The Richest Man in Babylon and The Greatest Salesman in the World were by the same author. It wasn't, but who cares? It makes no difference. It's not worth pointing it out - almost everyone has an aversion to being corrected, so only do it if there's actually tangible gain. Otherwise, go do something more important and not engender the potential bad will.

I consider this particularly selfish and evil. If you know there's a place where someone's map doesn't correspond to the territory, you should tell them before they inadvertently drive off a cliff. Even if it would be a status hit to yourself. And you can't tell me that's a part of their map they weren't even using, as they had just used it!