| Quote #1
Although she was a widow (her husband had died only the preceding summer), Mrs. Frisby was able, through luck and hard work, to keep her family—there were four of them—happy and well fed. (1.3)
At this point we don't yet know how tough as (tiny) nails Mrs. Frisby is. It seems like this kind of underestimates Mrs. Frisby, who we later learn has a whole bunch of tricks up her sleeves… if she were wearing any.
| Quote #2
She climbed up the tunnel, emerging whiskers first, and looked around warily. The air was sharp, and there was frost thick on the ground and on the dead leaves. (1.7-6)
It's not only the air that will be "sharp" if Dragon gets Mrs. F. The narrator makes sure we know just how much danger she is putting herself in in order to save Timothy.
| Quote #3
She looked at the sun; it was setting behind the trees. She thought of Timothy, and of the medicine she was carrying. Yet she knew she could not leave this foolish crow here to be killed. (3.22)
Foolish is the right word to describe Jeremy, but hey, don't we all like shiny things? What matters here is that Mrs. Frisby has done a favor for the crow, and Jeremy definitely repays Mrs. Frisby later. That's friendship.