Some points need to be clear.
1. Did you pay her the salary for the full month even when she was not able to work , due to injury and treatment? That is needed as a normal human courtesy and a liability as the master or employer.
2. Did you not take up the entire expense for treatment? It is normally expected of you because the injury happened at your home when she was on duty.
If the answer for the above two is 'yes', then legally or technically you are not bound to compensate further. However, to be on safer side, get the discharge certificate from the hospital stating about the issue and stating that the patient is now fully cured and is able to take up all routine activities which she was doing before such injury.
Keep the receipts, prescriptions etc. properly.
If you feel that she was very good to you all these days, as your magnanimity and human kindness, you may give her one month salary as advance or gift. I do not think they may ask for more.
If they insist on more then tell them that you do not want her service and pay one or two months salary and get rid of her. But make your moves very diplomatically of you suspect that they are trying to blackmail you and coerce you to pay more. Make your side clear by making your neighbours know about the treatment you gave and money you gave. Get signed receipts from the maid for anything you pay. Secretly record their talking to you as video clip or clear audio clip. You may tell them that you will give the money by crossed cheque or by direct credit to her account.
However if they do not have any bad intention, but need money genuinely, then be lenient, you may help them partially as gift and partially as salary advance.