Try to go to the bathroom immedaitely following sex. That is the best way to prevent a UTI after sex. Water always helps to stop urinary tract irritation. Also try Azo Standard, a urinary tract analgesic. It helps to stop the pain. I have found the best way to prevent infections after sex is to go to the bathroom ASAP following sex. This has prevented me from getting them for the most part. Somethimes you can be irritated by things in the diet or not drinking enough water. If it is irritation, then the AZO Standard should stop the pain within 2-3 days. I find that if the Azo Standard doesn't stop it by then, it is an infection. You can buy it at the drug store if you haven't already.
The UT symptoms are usually caused by an infection or something that has irritated the urinary tract. Certain things in the diet can definately cause irritation but not an infection. If there is a UTI, then there is an infection of the urinary tract. If she has become resistant to antibiotics it is because of just that, she is resistant to the antibiotic. Meaning the microbe causing the infection is no longer killed by it. Therefore an antibiotic that the microbe is not resistant to (or not killed by) is needed.
I'm pretty new to the IC game myself, but hopefully I can be of some help. To my knowledge there is a surgery available, but it's used as a king of last ditch effort after all other methods of treatment have failed. One of the things I've found that helps is regulating diet. Drinking lots and lots of water helps, especially the way you describe getting the UTI symptoms after having sex. I usually drink a tall glass of water as soon as I finish having sex (like within 5 or 10 minutes if I don't immediately have pain). A lot of times, if I drink enough water I can get the pain to go away without having to go to the doctor for an antibiotic. You should try to keep your diet in check... sugary or acidic foods are particularly bad, along with refined carbohydrates like pasta and white rice. Stick with foods like vegetables, low sugar fruit like watermelon and pears, potatoes, and chicken. Basically, whole foods are the best for you. The more you stick to this kind of diet, the better. I've also been told that IC medications often need to be used in combinations or need to be switched around. Talk to your doctor and if he/she doesn't seem as helpful as you would like, find another doctor. Oh, and one last note, the UTI symptoms, at least I've been told, are only symptoms instead of actual infections. The IC causes muscle contractions and those contractions cause the irritation and UTI symptoms. That's probably why the antibiotics stopped working for you... at least I think. Again, I'm new to all of this, so hopefully this is helpful to you. Good luck!