Ironically, if Nina reunites the two, Eris will be really grateful and want to be friends. Also, the fact that they could meet so easily just shows that she could've just talked things over with Rudeus, and then they could've just agreed to take a break separately to train, and maybe sent each other letters or something. No need for the amount of melodrama the author chose instead. Just bad / lazy writing.
Not like couples in real life don't mutually agree to live apart for a while to go to school or something, get some job training, etc.