Occasionally a person is offered an opportunity that is too good to refuse, so they quit their job and jump into the new venture.
But sometimes things are too good to be true, and the employee may want to come back. And this is a great opportunity for the employer. What kind of employee was that person? How did they leave? Think about how they acted their last days; that will give an indication of the type of person they are. Be sure to communicate any reservations you have to them and listen to their response.
Hiring an employee back that had previously quit can be a great benefit to a company, if it is approached the right way.