D-Mannose is expensive, but taking it daily would help I think. You also need to take fairly large does, more than the bottle recommends certainly.
Another thing that really has helped me is probiotics. I like the Jarrowdopholous (spelling?) brand. You can find it in healthfood stores in a cooler, and I buy the yellow bottle. I'd have her take a lot, like 4 pills, 3 times a day for about 2 weeks, then she could go to 4 pills twice a day for maintenance. Once the problem seems to be gone you could go cut that dose down a bit, but I personally plan to take them forever, as long as I have access to them. Actually, it has been shown that there is a strong relationship between gut/urogenital flora and UTIs, as the other poster pointed out, so this is an important area to work on. Here is an article that discusses this issue: