Ellie,
I'll bet if he justs ups his H2O and does some short walks during the day {@ break times?} and phases out any "bad" stuff he may be choosing to eat {pop, sweets/salty, booze etc?} - I bet it may just drop off. It's proven {somewhere-LOL} that many short walks is as good as one longer walk.
maybe a pedometer?/ so he can see how many steps in a day and increase it little by little?

Men generally have a faster metabolism than us gals- their muscles are programed to burn more calories than ours.

I know if my hubby stops the beer/snacks and the skips a few big meals he trims right down. sickening. { errggh not fair -just doesn't work for me!}
He does walk a lot at work though too.

That diet MIL sent sounds like a fad type diet - not a lifestyle change.