If he is running under Pow, there's no reason to be running 'script/server' or 'rails server'. That's what Pow does for you.
As with nearly all Rack application servers I know of, you can simply restart the application by creating/updating 'tmp/restart.txt' from your Rails root. Unix's `touch` command does this nicely. With Pow specifically, you can also create '~/.pow/restart.txt' (or '~/Library/Application Support/Pow/Hosts/restart.txt', since it's the same location) and that will force Pow to reload itself.
Also, I've tried that and I'm still getting the same old "Action Controller: Exception..." error. Specifically I have a MySQL error, yet no 'better_errors' showing.
then start the server again: rails server