You can treat a mouth rot on a turtle by washing it with a mild antiseptic wash, along with watchful feeding and maintenance of a clean habitat as well as trying to isolate the animal. However, a veterinary's advice ought to be sought at the onset.
This is very common when the turtle lives in an unhealthy environment or pieces of old food are around. You have to clean her habitat better. There is a medicine for mouth rot in reptiles but you want to take him to a vet and then follow his indications.