First of all, you need to take her to the vet. Sneezing is the first sign of a respiratory infection, and pet store rats are notoriously prone to them. Make sure to take your other rat, Baby, as well! Since quarantine was not done, it's possible that Baby will also get sick from Buddy.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry