Snowbell: You've got guts, kid! And you've got spunk! Not to mention moxy! You've got guts, spunk, and moxy.
Margalo: You're as big as you feel.
Stuart Little: How can you think of eating at a time like this?
Snowbell: Look, I'm nervous. And when I'm nervous I eat. 'Cuz I know, in my growling gut, that if anything happens to you, I'll be blamed. I'm sure the Littles already know we're gone and are planning to replace me with a hampster.
Snowbell: I wish I were the one that was dead.
Margalo: Really?
Snowbell: No. But I'm feeling very unhappy.
George Little: Dad?
Fredrick Little: What?
George Little: Am I in trouble?
Fredrick Little: No, son. You're in BIG trouble.
Falcon: I'll be back for you, furball.
Snowbell: Don't hurry.
Falcon: You should have stayed in your little hole, mouse-boy.
Answer: Probably because he could see that she was hurt at what she did and also because earlier in the movie, he said "When you're friends with Stuart Little, you're friends for life". Also if you wanna look at it another way, him helping her could be payback for her saving him from the sink. It's just a long term friendship they have.