By the time they reunite Rudeus will have his own life (even a kid with another one on the way) and all Eris was working for seems to be for nothing. Redemption is spoiler: However this is a fantasy story, so when she returns to him (he wrote her a letter asking to join him, he investigated and knew after a time where Eris was - this is where its most obviously a bad writing and that Eris is just a convenient plot device to save Rudy, sadly

) its just the exact time he is having his rematch with Orsted and she will not be a burden for him. And this is the exact reason why she will be accepted back. Because it's only fantasy. :-)