the other factor here isn't as much the carbs in the FF, which i think is around 4-5%, but the volume and timing. Most people give most of the food to a cat in the first 4 or so hours of each cycle and minimize the food after +6 or so. Some cats will graze evenly throughout the cycle, which works fine, but for her to inhale an entire 3oz can after the nadir will definitely increase her blood sugar. the Lantus' effect is strongest in the first half of the cycle and waning in the second half. Feeding her when the insulin is waning puts the brakes on the Lantus and can raise her numbers.
Try feeding most of her food early in the cycle, and perhaps just giving minimal boiled chicken treats (1/2" cubes) after pokies in the second half of the cycle. Anything pure meat has 0 carbs - freeze dried meat is another possibility for treats.