The 5th circuit decision addresses money not stuff like food and some of the commentary clearly cross references that in TX that the payday law also applies.
We don't know what if anything ops spouse signed nor if employer docked her pay/tips or merely jawboned server to pay
The issue OP's wife has is that the till is short. She happens to know that the till is short because certain customers did not pay. The issue is not food. After all, the same thing could happen at a the hairdressers - a customer could dash away without paying, and if the owner tried the same thing on one of the employees, we wouldn't say that it is about the dye or whatever products were used on the hair. The raw products used in a meal are far less than the cost of a meal, otherwise rent, utilities, salaries, advertising, etc. would never be paid. OP's wife is not being asked to restock the pantry, she is being asked to pay for her customer's meal at the cost listed on the menu.
The employer expects to take the difference out of her pay. Whether or not she signed something, if it was a condition of employment, then it is not "voluntary". It is only "voluntary" if not doing it is an option.
I think that we can agree that OP's wife should file a police report if her employer is unwilling to do so.