Did she sign a contract with you? if not it is considered at will employment meaning she can quit or u can let her go for any reason whatsoever without disclosing the reason why. if she did sign a contract read it and see if she is in violation of any part that will allow u to fire her if not let her ride out the contract and dont renew it. now for the handpiece i doubt it would be fair to take it out of pay but since i dont know the whole story i wont comment on it other than to say if u do take it out of her pay u should do so before u fire her otherwise if u keep her last check as recompense u could face an enquiry by the bbb and medical board. u would probably be right in doing it but why take the chance.
