What's the situation? Is she living alone? Does she have any carers going in daily either family/friends or official?
It's very common in the elderly and as pps have said there are behavioural reasons why it happens.
I'm not even elderly and I'm guilty of doing the same! Because it hurts me to move and going to the toilet is problematic for me I'm reluctant to drink but I make myself or try to. I have some drinks in my room as well as in kitchen to try and motivate myself to drink more.
They're reluctant to drink because fetching a drink is a pain (literally) also because they get fed up going to loo a million times a day, which may also be causing them pain.
They may also be worried/embarrassed about incontinence issues. Which ironically are more likely with repeated uti's. So honest conversation and address the issues here. Consider incontinence pants (actually quite comfy) or other equipment.
Making it easier physically to have drinks - a flask of hot or cold preferred drink or jug of water placed easily to hand, lightweight mugs/cups to use, mobility aids to make getting drinks, getting to loo easier.
Also consider high water snacks, melon, grapes, oranges, soups, even ices as a way to get more fluid into them and stimulate their desire to take in more fluids.
Is the loo easy to use/comfortable? Does the seat height need adjusted? Does she need a handrail? Has she got to go upstairs to go? Would a soft seat be helpful?
What's the situation for needing loo when in bed? Is it hard to get out of bed, is the loo downstairs and she is put off by that?
Lots to consider.