Oh wow, this has made me feel so uncomfortable now. I'm (thought) friends with my hairdresser and we get on really well, I also work in the beauty salon based in the same building so see her twice a week there. I've booked her to do all the hair for my wedding and then said "you and you BF are more than welcome to come the the wedding as well".
Now I'm completely doubting myself...
However back to your problem; if you actually genuinely like her (you don't have to go out clubbing with her to be friends) then just be friends with her! I know you have to remain professional, but if that means you can't become real friends with your clients then that's a level of professionalism I'm not willing to go to.