Is it permissible for a fasting person suffering from almost complete dryness of the mouth and throat due to cancer treatment to moisten their throat without eating?

Water reaching the throat breaks the fast of a fasting person. A sick person who is affected by fasting through an increase in illness, a delay in recovery, or excessive hardship is permitted to break their fast, due to the Almighty's saying: "But whoever among you is ill or on a journey [during them] - then [he is to fast] an equal number of other days." So, if fasting is difficult for you, or increases your illness, or delays your recovery, or causes harm such as choking, then you are permitted to break your fast. You must make up for it in the future if you are able. Otherwise, you must feed a poor person for each day you broke your fast.