Example 1: Alayna is your employee and she deals frequently with customers and the public over the phone. She does her job well. You discover that Alayna also has a job as a server at a restaurant known mainly for its scantily clad waitresses. Should you ask Alayna to give up her job?
No, what Alayna is doing is lawful, off-duty conduct and some states have laws that protect employees from discrimination by employers for such off-duty conduct. Depending on the business you're in, in very limited circumstances you may have a bona fide reason for asking Alayna to give up her other job.
Example 2: Zachary is your employee, and his work hours are 9:00 a.m. to 6:00 p.m. Zachary also works as a card dealer on a casino gambling boat. Zachary has been late in getting to work quite frequently, and it seems to be happening more and more. Do you ask Zachary to give up his other job?
Yes, it's appropriate to ask Zachary to give up the other job because it's affecting his performance at your business.