He also doesn’t understand the Eucharist. The Eucharist is the Presence of Christ in the Bread and Wine, which becomes the Body and the Blood. Note that the Body and Blood retain the physical properties of bread and wine. The bread still has gluten and starch, while human flesh has lipids, the wine still has alcohol while human blood has plasma. It is not a chemical or physical transformation, but rather a spiritual one. By spiritual I don’t mean by any means a “pretend” or “merely symbolic” transformation. I mean spiritual along the lines of the Transfiguration, a supernatural event that we cannot approach scientifically.
No, what comes out of you is the residual leftover of what you consumed, the gluten, the starch, the alcohol. We do not digest the Presence of Christ, it can’t be thought of as a physical component, nor is it subject to the normative properties of consumption. The bread and the wine are only signs and substances of the Real Presence, and that Presence can come to us without - we only use bread and wine because Jesus commanded us to do so.
Think of it this way, if it helps. You have a cheesecake. You can slice it, you can eat it. Let’s say you use it as a doorstop. So now you have a cheesecake holding open a door. It remains a cheesecake. You can still slice it and you can still eat it. When you digest the cheesecake, you’re not digesting the door stop.
He probably has less trite reasons, but is using the one he finds most ridiculous or humorous.
He’s phrased the question in a way that is difficult to respond to - similar to someone asking you “Can Mary save you?” This is a distortion of the reality of the Eucharist that we as Catholics understand. We don’t look to the Eucharist as a foodstuff or from a chemical perspective. We see it as a miracle, an act of supernatural Grace, and we accept it as such.