If you’re going to be a dick and correct the grammar of a bathroom sign, shouldn’t you also add the additional accents that are missing?
#la langue de Molière
About 90% of the time, accents are omitted on capital letters in French. By custom it is more incorrect to write an accent on a capital letter than it is to leave it off. That is not a “grammatical” error worth noting. He isn’t being a dick for correcting someone. The indirect object pronoun mistake was a pretty significant error and anyone who even remotely respects the French language should feel the need to fix it, for his or her own sanity and for the betterment of the writer.