That is an excellent question. I personally would never have done that, but good editor does not automatically equal ethical giant, so… First of all it is a conversation you can have with the editor in advance.
You can make it known that a career is not a single job and if s/he wants continued work from you (as all freelancers must constantly look to the next job), they are better off to continue to work thru you. Back when I was free-lance, I knew where my bread was buttered.
You can also make an editor sign a non-compete document. There were times when I had to do that. This just basically said that I wouldn’t steal a company’s clients. Usually there was an expiry date that said the editor could do business outside the company after they hadn’t worked with the company for after a certain period of time. I would usually modify that to include that I could continue to do business with clients I knew prior to working for this company.
In the end, the market isn’t so big that a single client is gonna keep an editor working full time, so their best bet is to work thru you if you can keep them busy…
Patrick