Pay attention,
I have a degree in Nutrition and Biochem...
yes, you can’t gain weight from nothing, but you can have a metabolism so slow and disease states so profound that weight loss is extremely difficult in ‘fast and easy’ terms.
Plus you have genetics, fat storing/famine hormones, aging, activity etc. It’s not ‘gee, just eat a little less dear and you’ll be a size 2 next week!’
You may have to accept that the smallest she can be and be healthy is a size 12, and it may take her a year or two to get there.
That’s the reality.
And the other reality is that if she’s married to an uncaring selfish lout, she’d be better off leaving him and THEN losing weight.