Hi. I am a dentist and the advice given by Bryanna is good info. Finding the communication or "oro-antral fistula" is pretty easy. You just have the patient hold their nose closed and create back pressure. Fluid will flow into the oral cavity and be easily seen.
Correcting this problem requires a sophisticate and skillful surgeon. The procedure involves opening the the gum tissue adjacent to the opening and then sealing up with a membrane, usually one that dissolves or resorbs with time. The problem is that the maxillary bone in that area is delicate and thin which makes healing unpredictable. It may take more than one try.
Your best bet would be to go to a dental school and show them the problem. You just might find a good and ethical practitioner who is willing to give it the time it needs.
Hope this helps and good luck
Topgum