Yes, it is legal in that the law does not require physicians to see an particular individual patient, especially if there is some personal reason to refuse to do so (like the fact that your mother had been using this doctor's husband as a doctor; Dr. Marie can legitimately feel it is inappropriate or would friction with her husband, etc. to see your mother as patient). Remember: doctors are in private business. Like any other business, they can refuse to take clients or customers for any reason which is not illegal discrimation, like racial discrimination.