Unless the employee leaves for a role (more senior, more pay, and/or more responsibilities) which at his/her current company simply isn't available or feasible.

I have experienced this from both ends, as the employee who outgrew my current role, and as a manager who my direct report outgrew their role. In this case, apart from trying to create a similar role (which often than not is impossible), the best you can do is to wish them the best, and be proud of the fact that you've worked with such an awesome colleague that they have now surpassed what the role could do for them. :)