Sorry it took so long for someone to respond to you. When a visible symptom occurs rapidly, such as overnight, as opposed to a long-term gradual appearance, it limits the types of problems that come to mind. Remember that your father has just had his immune system beaten up by his treatments, and a variety of opportunistic funguses, bacteria and virals can take hold without much resistance from the immune system. A fungal outbreak after the natural flora and balance of microorganisms in the oral environment has been disturbed by radiation treatments for instance, is a common occurrence. A condition commonly referred to as black hairy tongue is actually a fungal bloom and occurs quite rapidly, as does Candida fungus when the other organisms that keep them in check are destroyed by antibiotics, chemotherapy, or radiation. While it is hard to tell from your description if this is exactly what is happening, it could be a likely scenario. I would like to hear back from you to find out what this actually was when your doctors tell you. It will also help other readers of this message board who might be surprised some morning with a snow-white tongue (Candida Albicans) or something similar.