If she is self employed, the clients she has belong to her business, not your business, if she were employed then the clients would be those of your salon and your business and if you had a contract in place she shouldn't be able to take them with her.
That's why employed people in this business, on handing in their notice disappear from the salon pretty quickly and when asked where so and so has gone, they never know.
Contractually, with the not working within an x mile distance clause probably wouldn't stand the distance in a court and I'm sure most small business wouldn't take it to court because the legal costs would outweigh the clients who had left with the leaving employee.