I posted a question on mathematics.SE. English is my first (and only) language, and someone posted an answer which I could not understand to any useful extent. I do not doubt the poster's good faith, and I admit the possibility that what he posted was some kind of representation of a correct answer. My knowledge of geometry is limited—but sufficient, I believe, to understand a well-written correct answer to the question. However, I am certain that any attempt at correspondence with this poster to explain the present answer would be futile.
Somewhat sadly, I conclude that the least rude, while honest, response is just to do nothing. Is there a better response in this situation?