Why was “her” used here?
In accordance with studylight.org:
“…brother, in a boot makynge nettis. 20 And anoon he clepide hem; and thei leften Zebedee, her fadir, in the boot with hiryd seruauntis, and thei suweden hym. And thei entriden”
I think, that “their” should be used instead.
In Middle English, “her” means “their”. The Modern English “their” actually comes from Old Norse, while the equivalent (and predecessor) of Modern English “her” is “hire”.