That sounds very odd since you are losing both weight and inches. Also, to have it be 26.6% -- .6? -- both times seems off. Could be that those sites are your primary fat storage areas. There are additional formulas to calculate the percentage. I find the accuracy increases with the number of sites measured since it accounts better for differences in fat storage.
Perhaps she could use one of these other formulas here:
Also, if you want to eyeball it, these pics may help:
You also could ask another trainer to the body fat measurements, kind of like a second opinion, which is how I would put it.
If you live near a major university that has sports programs, they might have hydrostatic weighing, which is an underwater method. I have never heard anything about surgery to determine body fat. DEXA scans work, but they are very expensive and expose you to radiation.
And I think body fat percentage is far more significant than scale weight. Within the past year, the scale hasn't changed much but my body fat dropped another 6 percent and I went down another pants size. But, whatever you're doing, it's working. Keep it up!