Agreed.
If a puppy is being sold in a pet store it is either a) there for pure profit, or b) there because the owner of the puppy's parent did not get their dog spayed when they should have.
Yes, you can get healthy dogs from some pet stores. Yes, some of the people who have supplied the pet store with puppies may be good people.. but more often than not this is not the case. I would think that this would be especially rare to find in breeds like Chihuahuas. They are currently a dog that is being sold more as an accessory than a companion and people are looking to turn a profit on them. Your aunt is likely to spend way more than the dog is worth, if she were to buy it at a pet store, and is likely to have expensive vet bills just to clear up the problems the puppy would have from before she'd even purchased it.
All you need to do to see this is walk in to most pet stores. I know the one at my local mall literally smells of sickness. There is consistently loose stool in the bottoms of the display cases, the puppies' ears are usually filled with junk, and their is always discharge around their eyes. Not to mention that I've seen more than I can count that had nasty looking rashes (yes, I go to the pet store and play with puppies. I am only human).
In closing, my answer to your question is 'no.' Your aunt should look into the pound or a rescue for Chiuahuas rather than a pet store.