I find that just not credible. If he was taken away for questioning as stated then there would almost certainly be some record made, which would show up on passport scanning when entering. If that happened in the U.K. then a record would be made and it would lead to the person being stopped every single time they came into the U.K.(Assuming a non U.K. passport).

So I would countenance your friend to expect problems every time they travel to Sweden in future.