Regardless of carbs or not, she would have lost weight providing she was in calorie deficit. She was probably not giving you the entire picture of what she was eating. Unless they write down every morsel of food that enters their mouths, most people either seriously under-estimate their intake or seriously over-estimate it.
It reminds me of the dumbfucks who say "well i've been really good today so i'll treat myself to a pint or two at the pub." They subsequently undo the entire day's dieting. It's such a waste of time.
I'm amazed she was willing to give up carbs and go for higher fat so readily too. Usually "folks" are cluefucks who think fat is bad and carbs are health foods.
Most people like the one you just described don't realise that food intake is the major player in this situation. They seem to think cardio is what does the trick. I'd just let them think that cardio is magic and make sure they do enough to make the calorie deficit. I'd still push the diet of course, but if they're burning 200-500kcals every other day or so, even with the odd stray from diet they'd still expect to drop some weight to keep them happy, motivated and enthusiastic.