Lyle's SFP that is being discussed here is best suited to "stubborn fat." At 24% BF, the OP's wife should be able to use a variety of dietary and cardio techniquies to achieve her goals.
From
this cross-sectional study:
"Exercise intensity has been shown to be one of the most important factors in determining substrate utilization. We have demonstrated this to be the case in a wide range of healthy men and women such that, on increasing exercise intensity from low to moderate to high, absolute substrate oxidation can be seen to follow two patterns:
CHO oxidation continues to increase, whereas fat oxidation follows an inverted hyperbola. We have shown that fat oxidation increases from ~35% VO2-max to a maximal rate at an intensity of 48 +/-1% VO2-max. Further increases in exercise intensity lead to a reduction in fat oxidation."
"In our study, we confirmed that the crossover point occurred between 48 and 53% VO2-max, consistent with the 50% VO2-max suggested by Brooks and colleagues."
Another study derived a reqression equation to get from % VO2-max to % Heart Rate. The equation is: %HR = 0.64 * % VO2-max + 37. Based on that equation optimal %HR for FAT oxidization is between ~59% and ~68% with the maximal rate identified in the study at the upper end.