In 1395, why was "her" used instead of "their"?

Why was "her" used here?

Wycliffe Bible
Mk.1:20

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.

Solution 1:

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".

Solution 2:

You are right, for modern English.

In Wycliffe's time, the oblique cases of "they" were "her" or "hir", and "hem". "Their" and "them" were not widespread, though I think they were used in some dialects.