An employee of a Hand and Stone Spa in Franklin Lakes was arrested Thursday after he was accused of sexually assaulting a customer there while giving a massage, authorities said. The alleged assault was reported to the Franklin Lakes Police Department on Jan. 13 by the customer who claimed the incident had happened a day earlier, according to a release from the Bergen County Prosecutor’s Office. An investigation was launched and the employee, Kewin Sepulveda, 29, of Paterson, was arrested Thursday and charged with sexual assault, the office said. Sepulveda was being held at the Bergen County J…