My boys love mashed potatoes and I really try to make it a priority to not fix separate stuff for myself, so I try to be smart about it and use portion control. When I make them plain, I use red potatoes (about 6 depending on size), 1/2 c. warmed skim milk, and half a stick of Smart Balance margarine. No one at my house has said that they taste different from what we used to have in the past.
Sometimes I'll cook the red potatoes (6-8) and when mashing them I put in 3 cloves of garlic (actually, put this in the pot when they cook, then mash it in when you whip the potatoes), 2 T. of margarine, a quarter cup of grated parmesan cheese, and a half cup of warmed skim milk. With both, I let people add salt at the table. The extra ingredients here give it a good taste and we don't even use gravy or butter with these.
I use this site to calculate the nutrition: Recipe Calculator
Just don't say anything. What you FIL doesn't know won't hurt him!