What exactly is a mastectomy fitter?

Bra/Prosthesisexpert. Specially trained person in bra and external partial or full form prosthesis measurement and fitting swim suits and usually a great source of information on life after cancer - she maybe herself a survivor.
A helper. I believe that what you are describing is a person who herself may have had mastectomy surgery or someone who is familiar with a breast prostheses that a person can wear after mastectomy. The fitter would help someone pick the beast fitting bra and prosthesis after a mastectomy.