Shawn, I’d be curious to know who the client is, however, it seems that this is a question related to fuel utilization during exercising.
I am sure you are familiar with the respiratory exchange ratio (RER) which is represented by R. When R is 0.7 100% of the fuel to produce ATP comes from fat. R is 0.7 at rest. When R is equal to 1.00 100% of fuel required to produce ATP comes from carbohydrates.
It is important to remember how many calories are burned during a bout of exercise as opposed to how much fat an individual burns during a bout of exercise.
You might find it advantageous to get a really good university level textbook to read up on RER as well as EPOC (excess post oxygen consumpton) to see why training at higher intensities will promote greater reduction is body fat as opposed to long, slow distance training.
Hope this helps.