Hi. I think the issue is not with taste buds. In fact, after radiotherapy there develops a phenomenon we called Radiation induced mucositis (RIM). There is inflammation of mucosa of mouth and throat along with destruction of salivary glands that produce watering in mouth. This results in extreme dryness in mouth and pain while eating and even talking. There can also be ulcers in mouth. Unfortunately, RIM take long time to settle and dryness of mouth usually last quite long. There are many measures you can take. Ask your mother to take more fluids and semi-solid high caloric diets. Taking frequent sips of water along food help reduce dryness. She can use artificial saliva for reducing dryness. If she is not diabetic natural honey improves RIM and also have high nutritional value. Honey can be mixed in water or milk. Nutritional supplements like ensure powder will be helpful if she is not having diabetes(sugar). If despite above measures she is unable to have good diet, A feeding tube can be passed in stomach or small intestine. Even giving water and nutrition by injections can be considered as last resort. Hope I have answered your query. If you have any further questions I will be happy to help