not that I'm any expert here compared to the other women..
but i have found that when I have found that for me tis not just 80% diet... but more like 100% diet... and that i'm doing a weightlifting program 4 times a week.
Does she do weights or just cardio ?
( you said she did spin classes?)
i haevegone through periods in my life where I was doing lots of running/biking ( every day!) but never really lost any size.
Even when I did ONLY weights and ate well for 2 weeks I noticed a difference.( I should remember this in trying to keep to my own goals)
Look at competitive swimmers, and competitive short distance ( 100 -400m) track athletes.. COMPLETE difference in body shapes.
I used to do compeitive swimming in high school, training twice a day for 1.5hours - 2 hours at a time.. yet I was never really that small.. and there were plenty of girls who were fit and healthy, but could be flabby.
It wan't until I incorporated runnign and weights that I dropped the weight...
anyway, dont' listen much t me, I'm still learning!