Has this chin been seen by a vet? Unless you are extremely experienced in handling chinchilla illness, and unless you have an accurate diagnosis, then I would not administer any drug without veterinary supervision. Since chins rarely get intestinal worms, and that is what fenbendazole is most commonly used for, I would want to be absolutely sure that that is what is going on here before administering any drug.
If the chin has been seen by a vet, and had appropriate testing done, then your best course of action is to contact the vet and ask them for the correct dosing. They will have an accurate weight on your chin and be able to guide you accordingly.