S.P.
Because she is very very old, and perhaps not totally competent
in dealing with such things, one option would be to stop by to visit her,
asking how she is doing, and get a sense of whether she understands
this kind of thing.
Not to ask her directly about this particular situation, but in general.
If possible, find out if she has one or more family members
who helps her with these kinds of situations -- taxes, etc.
Then communicate with the family member(s).
In a kind way.
Another possibility is to look up her address in the County records.
See if her home is listed not just in her name as an individual,
but perhaps w/one or more other people, as co-owners, joint tenants.
Etc. And then contact the other person/people.
I'm imagining the mail from the water company lying in a pile of other mail,
unseen by her.
Good luck.