I've been digging around to see what information is available to define what constitutes 'attending a meeting', 'business activities', or a 'business visit' and what is excluded. And for the sake of interest, Germany (ie meant as indicative in Schengen in general, not Danish policy specifically) has published this:
'Posting of persons for temporary activities in connection with machines, trade fairs and exports' which might be most relevant for your colleague's activities, appears to allow audio technician work up to 90 days in 365 (not 90 in 180 like the visa waiver itself):
https://uk.diplo.de/uk-en/02/visa/profe ... 66-7-panel
It seems that the work exemptions are country specific across the Schengen. And I wasn't aware of the 90 in 365 restriction (though I guess this is 90 in 365 for a specific country, not Schengen in total), as compared to the 90 in 180 Schengen visa waiver allowance.
It also seems clear that unless there is a specific exemption, any activity beyond talking in a meeting/negotiating contracts/listening to speakers and very similar isn't generally allowed under the 90 day waiver.
For instance, even volunteering typically needs a visa:
https://uk.diplo.de/uk-en/02/visa/-/244 ... eer%20work.
https://immigration-portal.ec.europa.eu ... lunteer_en
The questions of what people tend to declare as they enter the Schengen, how these rules are enforced in reality, anecdotes about what people have got away with undetected, or what punters think should be the case are entirely different of course. I'm only referring to the legal situation.