All depends on the immigration status of your employee.
Any foreigner who leaves the United States Territory is subject to inspection and admission at the time he arrives to the US Port of Entry.
Any people other than a US Citizen, including Permanent Resident Aliens are considered seeking admission to the United States upon their return from a trip abroad. Permanent Residents of the United States seek admission at the port of entry presenting their Green Card Document and their Valid Passport; foreigners with an immigration status lesser than Permanent Residents of the United States seek admission by presenting their unexpired valid travel document and the Visa (except for those who are covered by the Non-immigrant visa waiver program).
A Person who holds a job in the United States must be authorized to accept employment. Employment authorization is tied to a particular type of immigrant or non-immigrant visa and could be a work related visa or family, humanitarian, or other type that confer this right once the application is pending or the application has been adjudicated and granted.
If a person has been present in the United States and has violated the immigration laws by being present without having been inspected and admitted, or by having over stayed their authorized period of admittance; or by violating any of the conditions of his or her particular visa for example accepting employment without having the right to accept employment, the person may have extreme difficulty obtaining a visa to re-enter the United States.
Any person who could face problems to re-enter the United States after a trip abroad must first consult with an experienced attorney to determine the possible consequences of a short trip outside the United States Territory before the person physically leaves the country.
There are different visas to enter the United States; ones are work related visas; others are family related visas, others are humanitarian visas and each one require the existence of certain elements that must be satisfied by the foreigner that permit the American Embassy to determine whether the person is eligible for that particular type of visa.
Tourist visas or visas for pleasure are the most common sought visas and are processed directly abroad.
If a person has work illegally in the United States, and the fact of illegal presence and illegal work comes to the attention of the immigration authorities (department of state consular affairs or inside the United States the Bureau of Citizenship and Immigration Services) the person will be found inadmissible under section 212 of the INA that set the class of aliens that are not eligible to receive a visa (immigrant or nonimmigrant) because of previous immigration law violations.
Some of the immigration grounds of inadmissibility have a waiver, that allows the foreigner to file a waiver with the BCIS that if approved will allow him or her to obtain an immigrant or nonimmigrant visa if otherwise he or she qualifies for the type of visa she or he is seeking to obtain.
Until the present, illegal presence or illegal work are bars to obtain a visa and the waivers are limited to those who have immediate relatives in the United States in the classification of parents or spouses.
If a person is not in valid immigration status or has violated his immigration status, that person shall not depart the United States Country because most probably than not, the person will trigger the 3-10 years bar of inadmissibility and will not be able to legally return to the United States of America.