She had cancer. She must have sensed that her time was running short and that she wanted her son to be with the natural father. It makes sense.
8 seconds sound clip from the Big Daddy movie soundboard.
You can hear this line at 00:33:44.823 in the Blu-ray version of the movie.
Quote context
[...]
- Mistake?
- I had a mother lined up for him, but she's banging the Pepperidge Farm guy.
- The kid just won't stop peeing and throwing up. He's like a cocker spaniel.
- I think he'd be better off with his natural mother.
- Well, she passed away last night, Mr. Gerrity.
- Group home? What is that, like, an orphanage?
- Yeah. We don't call them that anymore.
- Hey, hey. Look, I did it. Loop, swoop and pull.
- Jeez, you did do it. Congratulations, kid. I showed him that.
- Why don't you go back over there, okay?
- I'll put in a call to the Heatherton home.
[...]