I'm not sure where you live, but in the U.S., according to the HIPAA laws, the hospital staff cannot even tell you if someone is there or not ... or aren't supposed to, anyway. The immediate family, and anyone else authorized by the patient, is given a code that has to be revealed before any information is given over the telephone. I have gone through this experience with a sister who was in a Maryland hospital, and a brother who was in a Louisiana hospital.
But they didn't ask if she was a patient. The nurse said too much but if she had only said "she's having a good night. she's sleeping" I can't see anyone being fired over that. I think the part about her care (the IV fluids and the dehydration) was too much detail and why it would be a HIPAA violation. But the other stuff isn't professional but isn't firing material. People always ask how their family member is doing and "fine" or "asleep" is not a violation.