Hoarseness of voice is a normal and common side effect from radiotherapy treatment. Depending on which part of the tongue your friend was treated at, the larynx (voice box) could have been irradiated, affecting the voice. Because radiation continues to work even after treatment is completed, the side effects may get worse before getting better. Generally it takes 2-4 weeks for side effects to subside. But depending on the dose of radiation your friend received in the larynx, the voice may or may not return. It is best to discuss the issues with the radiation oncologist to see if the hoarseness is permanent or not.
As for the mucous secretion in the throat, that is also from the radiation treatment. Usually when the head and neck area is treated with radiation, the glands that normally produce saliva are damaged. This causes less saliva to be made, in turn making the saliva really thick.
To relieve the throat, try drinking flat club soda. There is also something called a baking soda mouthwash. Add 1 teaspoon of baking soda into 2 glasses of water. Mix it up. Rinse the mouth with this mouthwash 4-5 times a day. This would make the saliva not as thick and will make it easier to swallow.