Mmm it's a difficult situation this one. Here's my two pence worth....
Your running a very succesful salon by the sounds of it and you and your staff all get along nicely which is how it should be seeing as your with each other all the time.
We're all a little nervous when we start a new job when we are trying to fit in and get along with new staff.
Whether this girl has problems or not it should not reflect in her personality at work. If it does which sounds like it is then I'm afraid she's in the wrong job. In the hair and beauty industry you need to have good people skills ( good communication skills) which she doesn't come across as having.
There's times when I've had a bad day, felt Ill, had a row with my oh etc but I put that to one side when I get into work and put my professional head on and smile through even if I'm feeling like crap. She may be good at her job, practical wise, but if she hasn't got good communication skills then that's no good to you or your business. She may put your clients off and make them feel uncomfotable the way she comes accross.
You also have to see it from your clients point of view. Would you like to walk into a salon and be approached by a therapist like this? How would this make you feel? It would put me right off.
As for taking extra long breaks... well sounds like she's taking the p....
She should be impressing the hell out of you if she want's to keep her job.
It's your decision but if I was you I would do what has already been suggested and sit her down for a chat. If she doesn't change and carries on the same way then your salon is not the right one for her and you would be better off letting her go and hiring another therapist who is much more suited to the role. Hth