I would cite the cost of replacing batteries in the laptops, the cost of keeping an apple OS up to date, the extra cost and smaller amount of features available in apple office software, the extra cost of servicing an apple computer, the extra cost of purchasing replacement peripherals when the employees get fed up with the keyboard and mice, the extra cost of training for all people switching from a windows environment, the extra cost of buying extra drivers (or time involved with finding, tweaking, and installing) to allow the use of cross platform storage and many older peripherals, and finally, the extra cost of the damn computers.
I would also like to note that it is probably best for him to lease computers, that way when tech changes his investment will not have been wasted.
It is also a common misconception that apple computers are better for photo and video editing. There is a better video editing program only available on apple computers, but in the case of photos, photoshop has windows native versions and due to the better cost vs specs of a windows machine they perform better photo editing.