One more thing. You mention this as a slow cooker mac and cheese. Corn starch, flour, egg will all fail at some point in extended cooking. how long were you cooking this in your slow cooker?
Another twist is that slow cookers are hotter now than they used to be for food safety reasons. If she's using an old slow cooker, mine's over 20 years old, and you're using a new one, you could have problems with it getting hotter than the recipe was written for.