You're 3/4 right, they probably wouldn't gain weight, their good cholesterol would improve, but bad cholesterol could still be an issue. Years ago I read a book, I think it was called "Fit for Life" by Harvey and Marilyn Diamond, I recommend it for everyone. I don't try to follow it to a T but there is damn good advice there. Basically if you do brisk walking and climbing stairs for 11/2 hours per day, eat lots of vegetables, stick to skim milk everything else will look after itself.
On a scale of 0 - 10, McDonalds being 0, K.F.C. would be about 0.5 and Subway would be about 3.