Well rats are quite intelligent for a rodent and from what I've observed they do in some basic way learn from other rats in the colony so the nervous one seeing the calm one getting attention and treats, it may just work to get her interested enough to overcome her fear of you. I know most of my adult males all learned a single action that my retired big male Casanova does simply by watching him and watching me react to his antics.

Their big water bottle is wired onto the side of their big plastic tub by a wire that runs through two very small holes. When I remove the bottle to refill it, the wire goes a bit slack so Casanova sort of invented this "game". He stands up and tugs on the wire from his side, I tug back. Just a silly thing him and I do daily. After awhile some of the other big males figured this was pretty cool and that Nova was getting all sorts of attention for this so they now try to push each other around to see who gets to tug on the wire with me. Really cute actually other than I have to be really careful when I'm putting the bottle back on and tightening down the wire that I don't catch a rattie paw.