Author of the article here. Since I first published this blog post I was getting messages from people asking how it ended. Sadly it's pretty anticlimactic as the owner of the place had a meeting with the guy who put the Pi there (without me as he didn't want the Pi-dropper to feel ambushed) and in the end decided not to escalate it to legal and just basically told him to pack his things and get out. So no legal after…
The problem with this is you have no idea what harm the guy actually may have caused; nor what other RPis he may have set up around the company or around town. Next time he may be more careful with his username, set up the disk to be encrypted w/ a network key, &c, making future exploits more difficult to track down.