It was kinda more complex that that. 1. Hijiri said he wouldn't do it. 2. There was the insurance money (so the excuse about Hijiri owing money disappeared). 3. There's the illegality of the stepfather actions. 4. What's more, Nobuo was determined to help Hijiri. So the stepfather realized his "power" over Hijiri vanished and decided to "set him free". He knew he was gonna go under lots of (legal) problems otherwise.
D1alex explained it perfectly. This entire time the stepdad was able to push around Hijiri because he knew he wouldn't be able to fight back. Now that Hijiri has support, it'll be dangerous for the dad to continue to push, especially since he knows Nobuo and Hijiri won't back down.
The problem with the stepfather was resolved a bit to fast for me:
Hijiri: i don't want
Stepfather: okay i don't to see yr face again
End.