Her roux advice is, predictably, wrong. You don't specify a standard quantity of milk FFS, because you'll want it thicker or thinner depending on the dish. For macaroni cheese, it'll be a little thinner; for croquetas, it has to be solid. And for lasagne...
ETA: plus, if Squiggle's issue is a grainy cheese sauce, the issue is likely that he's adding the cheese while the heat is on. Turn the heat off, then add the cheese. But Jack doesn't know this, and in fact recommends turning the heat UP when adding the cheese because there is not a single cooking process she won't duck up.