I don't generally come to this forum so didn't see your post. Since the uterus helps to keep the bladder and bowel anchored, hysterectomy can lead to prolapse of bladder and bowel.
Does she really have a prolapse or maybe some other cause for frequent urination? As women age, frequent urination seems to be fairly common even without prolapse.
If her bladder is truly prolapsed, a pessary can be used to suspend it (along with the uterus). There are some that don't have to be changed very often. There are different types and they have to be custom fitted. But this would be preferable to a hysterectomy that causes a whole new set of problems plus is major surgery with risks for surgical complications (on top of the nasty after effects). This article is helpful in understanding the after effects of hysterectomy. Here's one article about pessaries; there are some other good ones out there - https://familydoctor.org/vaginal-pessary/.
Best of luck and please post back as to progress.
If your mothers uterus isnt prolapsed and isnt pressing hard down on the bladder due to other factors including being heavily antroverted etc then there is no point in having a hysterectomy plus if a uterus is taken out without suspending the top of the vaginal cuff then there is more chance of vaginal prolapses happening and there are a few types of vaginal prolapse including the bladder urethra, small and large bowel as well as uterine prolapse or vaginal cuff prolapse.
I would make an appt to see a urogynecologist as they are the prolapse specialists and also see a womens physiotherapist to assess pelvic floor strength.
I wear a vaginal pessary to support my prolapses at the moment and the correct type and size pessary is a good idea for older ladies who may not want an operation.
Take a look at all pessaries and get informed about them as not all the pessary ranges are known about by Urogynes, GPs etc.
Personally I am not keen on the whole woman site as the ideas on there seem very negative about POP ops etc and some of the ideas seem a bit off as well but each to their own.
I think it is important to get Urogyne advise as well as all options given both for internal vaginal support as well as strengthening of Pelvic Floor.
Speak to your Mums GP about suitability for vaginal oestrogen cream as this can help with vaginal atrophy and improve the vaginal skin