I don't want her to go out of business...?

ok, so my grandma has a very well known business. She has a worker that does a lot for the business (its alterations and the worker does all the alterations) it turns out the evil worker wants to go work at another place on the same street, therefore draining my grandma out of business. the worker has been stealing her customers by telling people to pick up articles of clothing at her house, and then telling them that she did everything and they should come to her instead. Therefore, creating a secret at-home business. She (worker) has no contract stating that she can't leave the job and open her own business on the same street, yet how do we prevent this?