Usually, even with advanced directives that spell out no heroic measures or do not resuscitate, there is nothing to prevent treatment to keep the patient comfortable. Sometimes that is medication for pain such as morphine.
In the case of a hip fracture, if no treatment is offered, the patient may become unable to stand and walk and become bedfast. That may not be in her best interest in keeping with the desire to provide comfort.
Intervention may be acceptable if it reduces or eliminates her pain and keeps her mobile thus preventing more serious complications such as a blood clot or pneumonia. Surgery may be necessary to pin the fracture site for potential healing. Although the surgery is invasive, it may not go against the intent of her wishes.
You may want to consult with a lawyer or patient advocate. Usually the hospital or health care facility has a social worker or case manager to help families navigate these murky waters. The Council on Aging may have a local office in your area. They can also offer you guidance and counsel in making this decision.