i can't say for zurich- but here in lausanne i often have yoga classes in the park, and i know of a tai chi class that does the same. so far (knock on wood!) no issues and i suppose it depends on the park but it has been quite nice as people see others practicing they often ask and then join in the next time. if he is looking to make it a business, i'm not sure how that would work. i offer classes outside of the park so it is a bit unrelated, but if they were to simply practice as a group, i couldn't see a problem with it. i would say (but, again, i am not speaking from anything but my experience here- in lausanne) to go for it and enjoy the space
and if something arises you can deal with it
otherwise, i tend to choose my space in the park that is a bit secluded as it gives a bit more privacy for those practicing.