Can't remember Darkwing being a huge success here in Scandinavia... Oh, he wasn't disliked. When the cartoon was on, people watched it, and enjoyed it. But compared to Donald and the gang? Darkwing was small-fry. He had that one cartoon series and that was pretty much it. When it was over, we went back to Uncle Scrooge and didn't really miss Darkwing.
I think a European Darkwing comic would be fighting an uphill battle... I can't really see how it could avoid being seen as an attempt to copy the Duck Avenger.
Maybe the two of them need to see if a European publisher is interested in more Darkwing Duck comics. Both William Van Horn and Don Rosa have created stories for European publishers (although, to be realistic, that was back in the 1990s-early 2000s, so it may not be as common now).
Is Darkwing Duck popular outside the US? I never got the impression that he was, or ever saw any European Darkwing comics.
Darkwing Duck comics had been printed in Germany quite a bit in the 90s, and more recently there have been two issues of the Lustiges Taschenbuch "Premium" series (which also prints/printed Ultraheroes, PKNA, X-Mickey, Mickey Mouse Mystery Magazine and Darkenblot) dedicated to Darkwing (they contained all the Boom! comics and some selected older ones). Peter Höpfner already said there will be another one with the Joe Books material and probably more old stuff.