I work in a research facility in Germany. I have lots of co-workers that are from other countries, mostly part of some international collaborative research projects.
There's one guy from China who's in his office almost every day including the weekend and also stays pretty late. I live more or less next to our office building and often see him at his desk while passing the building.
While it sometimes happens that we work on weekends during urgent projects, this should be an exception. However, this guy was in the office even during some public holidays (where we aren't even allowed to work as far as I know) and I see him almost every weekend.
I would usually say it's none of my business, but I have a feeling that he does not really know that he's supposed to rest on weekends and so. I know working culture is different in China and probably he thinks that he's expected to work every day or similar. HR is usually pretty good in telling us what our rights and duties are when we sign our contracts, but probably he simply missed this information. I assume that nobody is aware of it, since nobody else is here on these days.
We have fixed working hours that we should meet and we don't clock.
Should I approach this? If yes, should I talk to the co-worker or to our boss (who I think is responsible for having an eye on things like this)? I think I have a good relationship to both but don't want to step on their toes by approaching the wrong person and defying the other. I only want to make sure he knows that he really does not have to work every day.