Can a restaurant hold servers responsible for unpaid checks when customers leave without paying?

A restaurant could potentially do this in two different ways:

1) If the restaurant makes this a term or condition of employment, that any time there is an unpaid check, the server must make it up, they can do this without proving any fault or wrongdoing on the part of the server. They must simply announce that it is a term of employment, and can do it going forward from the announcement. Anyone who does not want to work under those terms would have to seek other employment.

2) Even without making it a term or condition of employment, if the employee caused the loss (i.e. the non-collection of the check) either by negligence (unreasonable carelessness) or intentionally (e.g. let friends skip out), the restaurant could hold the employee liable.

Even if the employee is liable, the restaurant cannot take the money out of his or her paycheck without employee agreement or consent; they could, however, fire an employee who costs them money, and they could also sue the employee (including in small claims court) for the loss.

If your wife knows who this party was, she could  potentially sue them for any losses or costs she incurs, again, including in small claims court.