Ask her why she would want to eat hogslop used for fattening pigs
Which is literally the historical use of skim milk:
Before World War II, skim milk—a byproduct of butter processing—was not sold in stores, but either discarded or fed to chickens, hogs, and calves as a protein-rich replacement for costlier animal feed.
Historically, skimmed milk was used for fattening pigs, and was recommended as "not only the very best supplement for growing pigs, but is of almost equal value for fattening purposes" as it "furnishes a complete protein" and makes the feed "more palatable."