It is legal is she signed an agreement when she started working there, otherwise, no, it is not legal because there was no explicit amount the said customer was going to purchase.
As brought up previously, he could have wanted a cut,perm,dye,etc. equivalent to over 200$(hypothetically) so the manager is only guessing what amount the customer wanted to spend, which is not legal.
What is happening though is that the manager is being "nice" in a sense of not firing her, and just taking the money out of her check. Its happened to people before, the manager will take pay out of their check for the potential amount spent because 1.)the employee will not fight it, or else risk being fired and 2.)teach them a lesson 3.)Make up for lost potential income/more of 2.
**** it up, and tell her to be glad she has a job. Everytime(almost) I get pissed at my job, I stop and think "Wait a minute..you have a job, you go to college, you have a car,etc.. Why are you complaining right now?"
Put it in perspective. If she sucks up this $14, maybe she might learn something from it, or at least make her think twice about blowing off a customer.