Disclaimer: I am asking this question for a friend, since he does not want to make an account here for one reason or another.
My friend works as an IT expert in quite a large company, in Germany. He is responsible for many of the systems there: coding, databases, sharepoint and many others. Suffice to say, he is a busy guy. He is NOT a manager.
He reports to a manager that basically handles the prioritisation of his work, manages the projects and so on - management work. He [friend] also reports his progress on various tasks to this manager. One other important thing, is that his company implements the "8 hours of your butt in our chair" policy, irrelevant of actual workload.
Recently, a day unlike any other had occurred: there was no work. Everything was working perfect, there were no user reported problems, there were no tasks left to finish etc. His next "big" project would start Monday (it was Friday), so he showed up at 8 AM and basically had nothing to do. He asked the manager, checked the ticketing system and checked his mailbox - nothing was to be done. So, after remembering that a project in about 2 months would require him to work with technology he's unfamiliar with, he decided to read up and test this new tech in preparation for his project.
After that day, he reported to the manager that since there was nothing for him to do that day, he focused on preparing for Project X using Tech Y, and sent a short report on what he found out during his testing.
Come Monday, and he received a formal written warning (an Abmahnung) for what was basically formally worded "slacking off the whole day, doing things unrelated to his current workload".
The question is: was the management in the right to issue such a warning and how can he best defend himself about it?