Would changing to a perineal spray or wash help prevent UTIs?

Another thing that helped my mom a lot was a daily non prescription cranberry supplement which a urologist recommended. Urilogist told us that drinking cran juice not potent enough and recommended a supplement called Ellura which you can order over internet; the urologist said that she'd had good results with that particular brand. It's expensive but it really helped my mom -(UTIs affected her cognitively, she would be confused and defiant which was unlike her)

I don't know if peri wash would make a difference with UTIs, but I do think it makes a difference if there is any problem with diaper rash and skin breakdown. I disliked using products that are not designed to be no rinse in any kind of sponge bath because I know that with soaps and body washes there is always a residue left behind.

It is possible that if the soap is not fully rinsed off it can cause additional irritation.
There are a lot of things that can make the conditions perfect for a UTI.
Improper cleaning
Not enough fluids. I gave my Husband cranberry juice often and I think we had only 1 or 2 UTI incidents in 3 years. I would mix it with OJ or cut it with water and he would drink it all day. It was a bit strong as 100% cranberry.
Medications
not being changed soon enough or often enough
Poor technique in cleaning
facilities that are not properly cleaned and or sanitized.
(Are many of the other residents getting them at the same time? If so I also wonder about the technique between residents)
And it seems some are more prone to them than others.
This might be worth having a meeting about with the administration. And if a Peri wash is something that you want used they should honor those wishes.

Also consider she may not be drinking enough water. Has she been checked by a urologist? Or is it impossible to get her to one?

Since she didn't have this problem before than I would say yes. You should not use soap in the vaginal area. Why not use wipes to clean her?

My mother has alzheimer's and suffers from constipation. For about 4 months she was in an assisted living home, hospital, rehab, assisted living home. During that time, she had to be on constipation medications, so she would go between long periods of not having a bm and having large sometimes very messy bm in her briefs. To make matters worse she could not shower because she was recovering from surgery and later from bed sores, so cleaning her was difficult. I witnessed both at the assisted living and rehab facilities that she was not cleaned well at all, as a result she had 3 uti's just in that short period of time. She has never before then had a uti, and never since she has been back home. I still cannot shower her everyday because her wounds have not completely healed, and sometimes because she just plain refuses, but I clean her well and often.

The change to perineal wash would be great, but there still is the "wiping" to be done, and if you are not there every single time to do it--then it's all for naught.
The baby wipes are great. Buy unscented ones, largest that they come. Hopefully help her to wipe herself. This seems like CNA 101, good grief, we were trained to do this a 2 yo's..and having a lot of patients with UTI's would be bad news to the NH.

Definitely! As a geriatric nurse I would not use body wash on the peri area outside of a shower/bath. How could they possibly rinse it well?? A peri-spray/wash and/or wet wipes (INTENDED FOR PERI-CARE, NOT hand wipes or the like) should be used. Especially on an elderly woman!  I have used wipes intended for adult care or even baby wet wipes, and they work fine.  I personally would use both, peri-wash/spray and wipes.  The peri-wash is great for tackling  bigger messes too, and will also help get BM or urine out of clothing as well. (Helpful tip!)

BatmannRobin: Why is the aide using a body wash (which is no doubt scented) inlieu of Cottonelle butt wipes?

I have read one should NEVER use body wash on "privates"....unscented wipes, front to back and cranberry pills should help. Also, if someone is taking antibiotics, it is also good to take PRObiotics, as antis can sometimes cause loose movements, which can exacerate the condition.