I can only help a little as the fistula I had was rectal only. Unfortunately, I had it for 9 years and had 20 surgeries before it was "cured".
Though I can't give any real clinical advice, the first and best thing you can do is take any complaints of pain, infection, bleeding, etc. seriously. The worst part of my care is an equal toss-up between the excruciating pain (thank God not constant) and the indifference/denial of my surgeons nurse. This made getting pain meds and appointments VERY difficult. I did write a letter to the doctor regarding her argumentative approach to my requests. As a result, I was dropped by the surgeon (the nurse is also his wife ) It was the best thing he could have done for me as the next surgeon is the one who cured in one surgery what he couldn't in eight.
I'm sorry I'm not a medical expert, but I hope having a little perspective from a former fistula patient helps. Though my pain wasn't constant, the flareups could be intense! Even the strong medicines didn't break the pain on a few occassions. Sitz baths as hot as I could stand them and reclining so I didn't put any pressure on the area helped. Sometimes, despite doing everything I was instructed and then some, Percocet was the only thing to get rid of the pain. REALLY HURT!
There are some good websites for information on fistulas (and most of them seem to cater to those with rectal-vaginal fistula). Even some of the patient discussion boards are filled with useful information. An example of one is emedicine.medscape.com. This is just one of many sites that may help you help her.
Good Luck and thanks for caring enough to find the best way to treat your patient.