No, if they suspect you of lying, then of course they're not going to coach you on what's allowed so you can say the right thing. That makes no sense.
Their overall reaction seems way out of proportion here, I agree. But the general tactic of questioning someone seems sorta like the entire point of having these agents in the first place.
Maybe I'm naive but I'd thought along the line that they could issue a warning that he needs a visa to enter and will unlikely be granted access without it. If he gives false information in the visa that process is probably the only option left - but in my opinion this still does not justify the treatment.
Perhaps I don't understand what you're proposing? What would they warn people? He had to go through ESTA and within 2 clicks from the US Embassy Germany's page on that, it says if you're being paid for performing you're not applicable.
