Was the questioner's eating of pizza, about which there is doubt concerning its permissibility, a mistake? And should he have refused to eat it? And does his failure to warn the young man make him sinful?

The basic principle regarding food is permissibility. Food is not forbidden merely by doubt. However, it becomes forbidden if one is certain of its impermissibility, or if there is a strong presumption of it based on useful indications. One must denounce whoever consumes it while knowing its impermissibility, or inform them if they do not know. It is worth noting that the slaughtered animals of the People of the Book are permissible.