Glad to see a voice of reason.  I was thinking like a bodybuilder:  "I use lots of whey when I'm cutting, probably more than when I'm not."  But that's because I like to consume my macros in specific quantities, particularly when cutting, and whey is a great way to achieve that. But you're right, the fatty sisinlaw will probably just start drinking protein shakes and not exclude anything else from their diet, leading to further calories in with no clear focus on moving those calories out.  So more weight gain is probable.