When a manager moonlights within the company, how do we pay her?

Q: Our salaried marketing manager (who does not have any direct reports) decided to work a couple of Saturdays as a bartender at our business to supplement her income, despite our policy discouraging it except in emergencies. It is her expectation that in addition to her salary, she will receive the hourly rate for hours bartending, and keep her tips. What is the best practice regarding managers receiving tips? Our restaurant managers [when required to fill in for absent workers] only keep their tips if they are the only one working that day; otherwise they usually distribute them to whichever tipped employees are working that day. Does the manager who is customarily required to cover for his or her staff get treated differently than the manager who does not? Are there other associated legalities I need to be aware of? – Heidi, Missouri