More than likely, it was her diet that caused the off colored feces/poop. Doubt that they were giving her the right kind of anything, from what you describe.
Children's chewable vitamin C is good.
Dried rose hips and dried papaya are very good for them and contains some good vitamins they need.
Alfalfa hay has much more in it for chinchillas than Timothy hay. Normally, you would feed an adult chinchilla Timothy,
IF he/she had been getting a proper diet. I normally reserve the Alfalfa hay for the females in breeding or nursing and for the kits up to three months of age, then I mix Alfalfa and Timothy half and half until the kit is 6 months old. For this one, I would probably give her alfalfa hay for a while.
Don't waste your money on liquid vitamins. If you notice, most liquid vitamins come in a solid plastic or dark colored glass bottle ... this is to keep the light away from them ... the light weakens the vitamins quickly ... plus, if you put liquid vitamins in the waterbottle, you really do not know how much they are actually getting.
A vet checkup with a vet that is familiar with chinchillas is a good idea also.
IF something is wrong with the new chin, it could be contagious, you want to make sure to keep them separated. I always suggest an isolation period for any new chinchilla, regardless of where they come from.
It's easier to prevent a problem as apposed to having to have to fix one.
You have become this little chin's earthbound guardian angel ... bless you ... and take good care of Ming-Ming!
Keep us posted!
Jo Ann