The “girl” was an 18 year old adult
Right.
Upon her 18th birthday, she instantly became completely responsible in all possible ways for every decision she made. Everything her parents taught her (for good or ill) became instantly irrelevant. She was a whole new creation, tabula rasa.
So ... her lethally stupid decisions were completely her own damn fault.
Paging Prof. Darwin ... Prof. Darwin, please pick up the black discourtesy phone.
Do you think she was paying for the surgery or was it her parents?
Yes but did she have the Adult Money to pay for her surgery?
few 18 yo adults can afford to pony up for elective surgery, so some OTHER adult with the cash is responsible for the decision to fund this ill-advised surgery, likely a parent.