Hi, Sorry your Gran is having such a hard time right now I totally understand your frustration i had 4 of them darn fistula's along my scar on my stomach. Does your gran have a stoma nurse that she can see? when my skin broke down and was red raw I used a barrier spray called Cavilon which helped the bags to stick also you can use a paste ring around the fistula and stick the bag to that it will stop the liquid running on to the skin both of these items were given to me by a stoma nurse.
If you post your message on to the Ostomy board you will get some other peeps who can give you some more ideas, as looking after these type of fistula's are the same as having a stoma. Hope you & your Gran get it sorted soon x 