It is actually in Chapter 3 of Mrs. Frisby and the Rats of NIMH that Mrs. Frisby sets off for home and meets someone. The character she meets is a very young crow who is in a predicament - he is caught on a fence by a string tangled around his leg.
The crow, who is very young, catches the eye of Mrs. Frisby as she races across the farmyard on her way home from Mr. Ages'. He is acting very strangely, repeatedly attempting to fly off, only to have his flight stopped by something securing him to the fence. Despite the fact that she knows she must hurry home with Timothy's medicine, she stops to help him.
The crow, who, despite his youth is old enough to know better, has picked up a shiny piece of string and got it tangled first with his foot, and then on the fence. Mrs. Frisby calms him, then begins gnawing at the tangled mess with her sharp teeth.
Mrs. Frisby is almost done when the crow calls out that the cat is coming. The last strand of string breaks, and, knowing that if he leaves her behind, Mrs. Frisby will be killed by the cat, the crow tells her to jump on his back. He then flies away, carrying her safely to her front door.
The crow's name is Jeremy. He is very grateful to Mrs. Frisby for saving his life, and promises to do whatever he can to return the favor, "if the time ever comes when I can help you."