Aww, she may be a wee bit thin, but she's far from being starved thin.
You aren't the first person to be the victim of bad pet shop information. The important thing now is that you are trying to change things. Whatever you decide to do, don't go crazy and try and feed her a jumbo rat to make up for it.
Just take your time, start out with something a little bit bigger than she is used to, and go from there. Or, if rats are out of your reach at the moment, just offer her the two mice, or even three if she's interested.
Gale