My co-worker is going to change jobs and it will be very difficult for us to get by without her, any advice?

This is one of the 2 people that work in my office that I am the manager of. Normally an office of this size needs more people than the 3 (including me) we had, but our clerk is awesome and can handle a much higher load of clients. But she has been offered a better paying position that will allow her to move up where she belongs. I'm encouraging her to do what her heart desires, but its tough to deal with the loss of a great employee and harder yet, a friend. Anyone have any advise on how to deal with letting go of people that mean alot to you in your life because it is better for them withot feeling any selfish thoughts about why they should stay in your life?