Actually, he might say that the Qur'an allows eating of everything except the animals mentioned in Verse 5:3. We say: Fair enough. Now show us whether it is totally disallowed to eat and drink our own filth, and how you deduce that from the Qur'an alone. If they say that "Allah does not command the Fahshah", we say that this would need an interpretation, since - believe me- there are certain religious cults that engage in drinking and eating human filth, so in their narrow understanding, the intake of such filth is not ritualistically abominable.