Boy, talk about a Catch-22.
In some cases, repeated UTIs, vaginitis and IBS problems are caused by an over-abundance of bad bacteria in the body.
The solution to this is to treat with antibiotics. The problem is that many antibiotics give you IBS-like symptoms (known as antibiotic-associated diarrhea or colitis). So you may correct the original problem, only to trade it in on a new one that looks just the same! Thankfully probiotics can go a long way towards correcting the problems associated with antibiotics.
If your wife also has a lot of GI tract problems, have her see a GI and get a breathalyzer test. It works just like it sounds: you breathe into it, and they can tell by the gasses you emit whether or not you have a bad bacteria problem, and they can get a general idea of how bad it is and maybe what bacteria(s) are causing it. Bad bacteria in the guts can be hard to get rid of, which means she would need yet more antibiotics, but they would probably put her on some much stronger ones and for a longer period of time. Someone on here said they knew of a doctor who didn't do less than 6 months of antibiotics for one type of bad bacteria! (Most, however, will give you 2-3 weeks.)
This should hopefully clear up both the bowel problems and the urinary tract infection problems. Make sure she takes probiotics during and after the antibiotic regimen to put good bacteria back in her system and keep the bad bacteria pushed out. There's only so much living space in there, so the more good residents you can have, the less room you have for the seedy ones to move in.
If she doesn't also have bowel problems, or she can't see a doctor soon, then see about getting her on some probiotics anyways. They're always a good idea while she's on antibiotics for any reason, but they may also help keep the UTIs from reoccuring. I'm only familiar with probiotics formulated for IBS (which certainly won't hurt her), but you may be able to find some at health food stores formulated for urinary problems. She can get results from any probiotic--including acidophillus milk and yogurt--but specialized products will probably result in a quicker, more noticeable improvement. Acidophillus milk is the smallest probiotic unit you can get, followed by regular yogurt, then yogurt for bowel problems (like Activia) and/or homemade yogurt; then you get into the pills, with, again, acidophillus pills being the weakest and Culturelle and some health food store products being about the strongest.
Four other things I can think of related to UTIs, and two are of delicate conversation. One, how you wipe your butt matters for a woman, believe it or not. Experts say that a woman should always wipe from front to back, not back to front. This keeps from getting fecal matter into your vagina or ***oris where it can cause an infection. Fluids from these two parts, however, don't contaminate the anus, thus why you go front to back. Secondly, sex can cause UTIs in women. But, in the article I read, they said that peeing after sex helps keep the rate of UTIs down. Something about sex can cause urine to stagnate or back up into the bladder or something like that. In any event, they found that women who peed very soon after sex had a significantly lower rate of of UTIs. Your wife might also consder washing herself afterwards to help keep away any foreign matter that her body may be obecting too streously to. Warm water and a washcloth should do it, but a bidet would probably work the best, actually. Supposedly you can buy bidet attachments for toliets in the US! A douche could also be used, but that could cut two ways. A woman's body naturally produces acid in the vagina just to kill anything foreign that gets in there. Douching too often rinses that away and can actually lead to a vaginal infection. At the same time, your wife has something that's not working right down there, so she might need the douche to do what her body is not (cleaning her out). Because, for a woman, all of those things are SO close down there, you have to treat all three very carefully because they can transmit problems to other places--meaning even though she's having UTIs and not vaginitis, her problem may really be starting in the vagina and moving into her bladder and urethra; you never can tell.
Thirdly, water, water, water. I'd say she needs a minimum of 64 oz, and probably more. In order to get my liquids requirement in, I make up a pitcher of green tea and sit it on my desk at work. It's so easy to just pour it out and drink it; no having to get up and go get more every half hour (which, apparently, I'm too lazy to do). I have also done this with water. Also, get 32oz or pint glasses to use in your house. That's all we drink out of at home and you automatically fill it up and when it's at hand, you almost always automatically drink it all. Only two of those will have her minimum requirement (I actually drink my 64 oz of green tea at work and do two more 32oz glasses at home with dinner--sometimes even more!). Fourth, she can try seeing an allergist or putting herself on an elimination diet to see if her problem is a food or drink allergy. I'd start with drink allergies (although food can cause a problem too). Cut out caffeine, artifical flavors, artifical colors and all forms of sugar (artifical and real). No coffee (not even "de-caf" which actually still has a fair amount of caffeine in it). In short, switch to straight water and see what happens. If the UTIs don't come back, then you may have found the problem. Add other liquids back in one at a time to see if they are tolerated. I would generally peg artifical sugar, flavor and dyes as the most likely culprits.