My sister was trying to get a VISA to come to his son's open heart surgery here in USA and she couldn't get it

My nephew is a 9y/o boy with a Cardiac problem. He is with my father here in USA. He has a VSD (ventricular septal defect) now his aortic valve is affected too; he needs an open-heart surgery. My sister wanted to be here for his surgery, which is February the 20th, she went to the American Emby in Mexico today to get a VISA (they call it a "humanitarian VISA"). We got a letter from the surgeon doctor who will make the surgery. She took the letter to the Emby in Mexico, but they denied the VISA to her, despite the fact that she was showing also a bank account, proof of income and properties in her name, She want's to be here in USA for his son's surgery. My father is going to pay for the surgery, so we are not asking for any government funding like Medicaid or anything like that. We are self-pay for the surgery. I don't understand why they don’t let here to be here for his son's surgery. (They told her she could not get a VISA because she may stay here in USA after the VISA expires) Anybody knows where we can get help, so she can be here for the surgery. It is really important because this was not planned he came for vacations and his health was getting worst while he was here so we thought would be better to have the surgery here in USA. So if she is not here, I (his uncle) and my father will have to take care of him after surgery) any suggestion is appreciated about what we can do to help her to come to USA for that surgery. Thank you