Ηey just saw the video as well. I think 30:26 is a better mark, I was also looking at the later part of their conversation and was confused.
I do agree with the second point Lobo made. These guys clearly have an established relationship where the boss is the more dominant party and can get away with such stuff. And that is the fault of Fassbender's character. Since it is not their first interaction it's probable this thing started slowly. Maybe with other dominant touches like shoulder holding, headlock hugging, backslapping and it escalated to this.
The thing is that simply accepting all the moves and doing nothing in return cements him as the submissive in this interaction, basically allowing the boss to use him however he wants. And no I don't think you should ever allow even someone with clear power over you to treat you like that.
Now I have never had a situation with a boss figure acting in similar way, but I would say you still have to impose your boundaries. And doing it early is the best solution, check the camel's nose metaphor as well. And what Lobo says is connected to that. If you have a more dominant and strong presence from the get go, people would think twice of invading your personal space.
The weaker you look the more people will feel allowed to take advantage of you, that's life. So in these situations you have to be firm. And if you get slapped at the butt you have to confront the guy calmly. If you don't, you basically accept it. James D gave a very good example of how to go about it.
And something last regarding the scene, I don't think that Fassbender's character carried himself well enough there. To me it feels like after the butt slap he immediately turned around saw the boss, and his reaction was like: "ah it's you boss? cool you can butt slap me all you want of course." He continued moving his head back and forth in a jittery way, never stood up straight or gave direct eye contact, while the boss had a pretty predatory underlook directly facing him.
The whole vibe feels a bit like the boss scolding him, setting him straight, and telling him what he expects from him in a not very forceful, but clearly dominant way. And I'm pretty sure Fassbender is a good enough actor that he intended to do all the little things in this scene to perfectly portray the submissive role.