I think the success of any weight loss scheme depends a lot on the 'mentality' (can't think of a better word - mind-set, perhaps) of those using it.
Some obese people, me included, never
feel full (it is a genetic/physiological thing to do with hormones) - it would be insanity to put me on a diet where carbohydrates could be eaten freely, especially if 2/3 of what I ate had to be those carbs. I know I would gain weight.
I lost 13kg on the 5:2 diet, mainly by cutting out carbohydrates, and more importantly, have kept it off for almost 2 years. This is the first time I have maintained a weight loss in 40 years of being overweight. I think that is just as important as the weight loss, in view of what usually happens with dieters - the moment they 'finish' a diet the weight starts going on again.
The 5:2 diet has taught my body that being hungry doesn't harm it - some food will be along eventually!
Consequently, even when eating normally, my portions are smaller than they used to be.
Edited to add - good luck, Grasshopper - I hope the scheme suits you and your mother!