I agree with Kristy. Vets do not learn much about chinchillas. They learn about more common pets. I know a vet in Maple Ridge told one lady to feed orange juice and banana chips to her chin to get her chin over a really nasty respiratory infection. As most know - antibiotics are what fixes those kinds of problems, not bananas or orange juice.
Even the vet I see regularly, while he is good for surgeries or doling out antibiotics when I need those, he does not know everything about their diets or care.
IMO, fresh fruit just is not a good idea at all or fresh veggies. You may hear something to the contrary of that, but I've seen and heard a lot of how that affects their digestive tracts, and some chins end up with really bad diarrhea or rectal prolapse (if the diarrhea is not fixed immediately from the moment it is seen.) from fresh fruits or veggies.
If you insist on giving fruit, make sure it is dried naturally (no preservatives or added sugars) and a small piece once in a while (once in a while meaning like once a week or once a month. Small piece meaning like half an inch or smaller in size of dried fruit like a cut up raisin or chip off a banana chip, or a small tiny cut piece of papaya.).
Moderation is the biggest issue with treats of any sort. Fresh fruits or veggies just are not for chinchillas though.