LARGE CYST on top of foot

My fiance has a 3″ round cyst on the top of his foot and refuses to get it looked at. I have researched the condition extensively and it has many symptoms of a Ganglion Cyst but is much larger that the typical pea size – 1″ described. It is firm but fairly soft to the touch and causes him mild pain, sometimes it is even temperature sensitive. He has had the condition for about 10 years and is in his early 20’s. He is not an athlete but is very healthy and works out lightly at the gym. He has never injured the foot. What is going on and how can I help him treat it at home?