Couldn't the manager/coach from the other team have asked for the game to be ended before the fifth inning? Could he have forfeited in the second?
Yes, but his team showed up to play, and should have had the chance to play without being humiliated. This isn't professional teams where everybody has the same shot at getting good players.
One thing that they could do is adopt the "mercy bats rule". If at the end of ANY inning a team is ahead by more than the mercy rule (10 runs in this league) the losing team gets to bat consecutively until they either hit 5 innings of batting (and the game is called by mercy rule), or they get within 10 runs, after which the other team gets to bat until they get back to the same inning or go up again by more than 10.
This doesn't prevent a run-up in a single inning, but does prevent these multi-inning blowouts.