have a emergency plan ready for the leaving of your key employee

Business is business, do not let the personal emotion get involved into hiring/firing decision.

Being a business owner, if I am in your case, there are two things make me concerned:

1. Who will take over the job of employee who just left so the company can continue to run?
2. How many business intelligent property the employee who just left have? Do he/she will use this business intelligent property against you? (for example, set up a similar business with same business model right across the street!)

It is very important to have a emergency plan ready for the leaving of your key employees. Be rational, not emotional.