Hidden Walrus is right.
It's so heartbreaking to see these guys, and we all sometimes can't resist trying to save one or another from a pet store. Most people start out by getting their pet rats from pet stores, because that's where they are readily available. I have found though that for folks who get bitten seriously with the ratty bug, they soon find themselves avoiding obtaining rats from pet stores, and either go with carefully selected hobby breeders (not backyard breeders), or through legitimate rescue organizations. They eventually just learn too much about the rat trade to feel comfortable doing anything else.
If you just have to rescue this poor baby I totally understand. Just to help you think it through though, here's some pros and cons for you.
You might save a life.
You might gain a wonderful pet.
Even if she doesn't make it, you'll have provided her with a loving, safe home for her final days.
Moving on though, no reputable hobby breeder would supply a pet store with their rats. Backyard breeders (through accident or irresponsible breeding), hobby mill breeders, and large scale pet trade animal mills will be where every one of these babies come from. Occasionally you'll also get pet owners who surrender their no longer wanted pet or the store itself breeding for profit as the Pet Supply Plus near my house does.
Overall though, you can almost guarantee that the babies come from large scale rat mills, which indiscriminately over-breed rats genetically prone to serious and chronic health and temperament issues in unhealthy high stress conditions.
So that said, Cons:
By buying her, you will increase pet store demand, and prompt the breeding of another litter to maintain profit. One rescue = Ten new babies born in the same conditions.
She will likely be too far along to make it.
Depending on what is wrong with her, even if she does pull through, you can expect to spend between $100 and $800+ to save her life, probably around $400-500, and she may be on medications for her entire life.
You can also expect a higher than average occurrence of tumors.
If you have any other rodents, you will need to quarantine her for 5 weeks to ensure she doesn't have a fatal communicable disease.
Depending on her temperament, you may spend a great deal of time working with her to make her handle-able.
It's not an easy situation or choice. My heart goes out to her and you. But hopefully this information will help you clarify the path you want to take. Best of luck!