Depends on whether it is worth the employer's while to delve that deeply into how funds are expended. It also then requires the employer to define what is an "adequate" breakfast. If one cuts the per diem allowance when breakfast is included, does that only happen if there are eggs, or is it muffins, or is it bacon and eggs?
The whole point of the per diem system was to get rid of paying employees to have their assistants enter data into a form and possibly scan in receipts and then paying people to review those forms and attachments. If one pays a per diem, it is simply calculated as $X times the number of days (or perhaps half days) of travel and that is the end of it. This leaves a hotel bill and perhaps some local transport depending on the nature of the trip. If all of this extra work can reduce the total cost of inputting, reviewing and paying expenses from $40 to $20 per submission, a few extra eggs here and there won't matter.