Does a business owner have the right to change their mind about offering services to friends for free?

My wife has a med spa and she had been giving treatments for free to a friend of hers. We invested in a new machine where treatments cost 3500 for three- her same friend offered to be the model for training. There is grey area as to weather or not she promised the remaining 2 treatments for free but regardless she decided she would charge the friend cost of materials- which is 175 per treatment. This covers the cost of the probe that is meant to be used only once. The friend said if the probe doesn’t break on the first treatment then you can just use it again for the second treatment- My wife said no, you have to pay for both probes. This resulted in the ‘friend’ getting upset that my wife had gone back on her word- I believe my wife reserves the right to change her mind even if she said at some point her friend would receive the treatments for free. Considering how much my wife had already given this ‘friend’ I was angry that she would not accept my wife changing her mind just to cover cost- let alone charge a discounted price