Michael,
First of all, its great that you are doing research on your own to help your girlfriend. That shows compassion and great kindness on your part. She is lucky to have you.
I am a bladder patient myself. I had recurrent UTI's beginning in my late twenties. Like your girlfriend, I was prescribed macrodantin, and then cipro over and over again. It helped for awhile, but the infection came back.
After a couple years of this, I finally got referred to a urologist. I was diagnosed with interstitial cystitis, which is chronic inflammation of the bladder. Basically its like having the sensation of a bladder infection ALL THE TIME. It also makes you more VULNERABLE to infection. Interstitial cystitis is often overlooked and misdiagnosed as just chronic UTI. The treatment is very simple: go to the urologist, get a bladder work-up (includes a cystoscopy), figure out what's wrong. If its interstitial cystitus, you get a prescription for elmiron that works by coating your bladder and you go on a bladder friendly diet which means avoiding things like caffeinated coffee, mayo, miso soup, and onions until you heal.
Urethral stricture (narrowing of the urethra) or a hole in the membrane of the kidney can also make a person more vulnerable to UTI's. It's hereditary in my family. A bladder work up can rule this out or get you on the road to fix it.
The thing about cranberry juice. Get the real deal. No sugar added. In the states we have something called RK Knudsen's 100% cranberry. It's not sweet and its very strong, cut it with filtered water. That ocean spray stuff is crap and won't do your girl any favors. Further word on juice, my urologist tells me that cranberry juice is actually rather acidic and you are better off with blueberry. Same benefit, less caustic.
The general Rx for people vulnerable to UTI
1.) pee before and after sex
2.) wash after sex with antibacterial soap
3.) 50mg of macrodantin every day you guys have sex.
Oh, and to make you an even better informed boyfriend lover: Read "She Comes First" by Ian Kerner. Expains a lot about how women are internally organized - from the plumbing to the branching of nerve endings that can make an orgasm happen in grand fashion or short circuit it out entirely.
Best of luck,
Carolyn