Hi,
I really can't comment on Tiro_Afila's suitability to dive without having seen him myself, but since this is an interesting thread, and since I'm an ENT doc, I just want to point something out...
Losing a mask is something that we all should be VERY comfortable with, and learn as part of our basic open water courses. Of course, if you inhale through your nose once your mask is off, the problem wouldn't be the few drops of water that might make their way from the nose, into the natural drainage pathway of the maxillary sinus (the ostium), and then into the mouth through the sinus puncture (oroantral fistula), but rather the massive flood of water that would simply go through the nose into the back of the mouth and throat. If you were to inhale into your nose and drown (or at least aspirate a bunch of water before controlling your breathing), the presence or absence of a fistula wouldn't have much to do with it, since there is a much larger pathway from the nose to the lungs in all of us...
If anything, an open fistula would help with equalization, since the pressure in the sinuses, the nose and the mouth needs to be roughly the same at all points during the dive. The sinuses are rigid boxes with bony walls, so any delay in equalizing the pressure difference (for instance, if you have a bad cold) will result in significant pain until the air finally forces it's way into or out of the sinus (depending on whether your are ascending or descending). I experienced this first hand recently in Anguilla - did I stop diving? No, but I'm a TERRIBLE patient...
In any case, the main reason for the fistula to be healed before diving is that any repair work done (such as a flap, or sutures) could be undone by rapid pressure changes that would be associated with valsalva or the spontaneous equalizing of sinus pressure...
It is also true that an open fistula can predispose you to sinus infections, but mainly from bacteria in the mouth. I suppose that some seawater could get in there as well (a less likely scenario).
It's probably best to be well healed before diving...!
Mike
