The best solution I read is to use small glasses . A 4 ounce glass or smaller is ideal. As they are easier to hold, are lighter and so harder to drop. And if they do fall, less mess is made.
Rather than asking someone to simply drink , one suggestion is to simply hold out the glass in front of them. This may receive a reply of, what is this? Or, I’m not thirsty. And so you then may feel the need to then explain why it’s necessary to drink. Personally, I would try and avoid that.
As far as urinary tract infections (UTI) go, it’s true that improved hydration does help flush out the system and so can also help reduce or maybe even get rid of an infection. However, UTIs often start because of incontinence and getting wet pants/underwear. This is all a breeding ground for E Coli and any other bacterial infection.
It is fairly straightforward for your GP to detect whether or not a dementia sufferer is dehydrated or not. Also the GP will be able to diagnose whether they have a UTI as well. If the dementia sufferer is being cared for at home, then I would suggest a regular GP's appointment in order to check for hydration issues and a UTI if any of these are a problem.