Nah, Kyla I don't think you can blame the cats. I've had outdoor cats for years, and never had a plant killed by cat urine. They don't pee enough at one time, nor do they do it in the same place. You'd be able to smell it if they did. Rain and irrigation would wash it away in any case.
Now, human urine can kill a plant for sure. Urea is a component of many fertilizers, but too much in one place can burn. That plant does kind of look burnt, with the crispy brown edges to the leaves. Maybe it got too much fertilizer? Or if you do have anyone at your house (it's usually the guys . . .
) who likes to 'go' in the yard, tell them not to do it in one spot, spray it around.
I don't know much about phlox, as they don't grow here so it's been years since I grew any, but I'd hurry and dig up the ailing plant, and take a good look (and smell) at the root system to see if there are grubs, fungus or some other problem underground? Otherwise, for something to up and die that suddenly, I'd suspect overspray of some sort of weed killer or even lawn fertilizer from the neighbor's "lawn".
You might actually be able to save it, if you rinse off all the old soil, let it rest in the shade in new potting soil for a week or two, and then plant it out again, somewhere else. OR, if you still have the receipt (I know, I can never find them) you could take it back to where you got it. Most places will guarantee a plant for a year - especially if you say you bought two, and one is still doing fine.
