I would suggest getting your chinchilla re tested and ensuring it is clear of the giardia. I have not personally dealt with it ever, but I have heard it is really hard to get rid of. You have to use a bleach/water solution to clean and disinfect their cages, you have to toss any toys or anything that the chin may have come in contact with, etc...
Try re testing the chin to see if they still have it. As far as I know, the fenbendazole is the correct medication to give. If the chinchilla is cleared of giardia but still having wet droppings, you can also ask your vet about using chloramphenicol to correct the diarrhea (if other options are not working like burnt toast or activated charcoal.). It works really quick on clearing up diarrhea.
For the meantime, always feed and care for your healthy chinchilla first, and your chin with giardia, last...to prevent passing anything onto your other chinchilla. I am not sure on the diet you're using, so will have to look that up. Using acidophilus can help though with the diarrhea and tummy troubles, though. You can give that in between giving medication to your chin. Say if you treat every 12 hours, give it at every 6 hours after treated with medication. This might help with the diarrhea too. Wish I knew more about it, but maybe someone else who has personally dealt with giardia can help you more so.
