“Maybe she had an underlying condition....”
No. A UTI can easily turn into Sepsis which is deadly!
I think older people might be more susceptible to it because they don’t feel pain as much in their internal organs like younger people do. That makes worsening UTI or appendicitis more difficult to detect.
There’s a self test by Azo you can buy at the pharmacy. It tests for two types of UTI. A couple times it has helped me decide whether to go to the walk-in clinic or not. Helpful but when in doubt, see a doctor.