I weigh my chinchilla every day. His normal fluctuation is usually 2 grams, which is at the error margin of my scale, although I've seen as much as 10 usually for no apparent reason. I think a good rule of thumb is +/- 15% fluctuation of average weight is a strong indicator of health problems. So if your chinchilla averaged 712g, and now weighs 25g less that is about 3.5% so the danger zone is around 100g. (Although personally I would take her to the vet if there is a 50g loss or noticeable change in fecal appearance).
You might see an additional initial decrease in weight and food intake for the next day or so as she will have to re-acclimate again.
As far as weather is concerned what matters most is room temperature. If it's fairly constant that should not be a problem. If the temperature at the cage was 70-8F and is now 40F that could explain the weight loss by itself.
I expect her weight will go up about to where it was over the next two weeks and suspect that it was a result in change of environment and stress.
Keep a close eye on her daily intake over the next 7 days and verify the food and hay looks and smells okay as well.
That is my non-expert opinion.
Thank you very much for your input.
As far as the room temperature goes, I think my friend's place have been around 60-70F the whole time, although it may be a little higher or lower on a certain day. My chin did lose another 5-8 grams after I got her back, hitting as low as 682g after I made this thread, but when I weighed her 10 minutes ago, she's back to 688g.
One other recent change is her food, she's been on Oxbow Chin pellets and regular Timothy hay all these years, but I just bought some KMS 3rd Cut Timothy Hay before I left her with my friend, and now she's eating a little more hay and a little less pellets, which I suspect is another contributing factor of the weight loss, but regardless of hay or pellet, she is eating a lot and pooping a lot
