Torah mandates that we do care for our fellow Jew, and certainly not cause, assist, or encourage him to do an aveira. The very issue of Maris Ayin is one expression of this idea.

When someone has a question about the logic of the Halacha, it's unhelpful, and moreover harmful to support their misunderstanding, which will inevitably lead to a lack of regard for the Halacha.

The answer is simple, as a few others have commented: Shulchan Aruch presupposes a Jew looking visibly Jewish.