"To the Bottom" quest not finished for host of multiplayer?

[Herr Bert]

I've been playing the multiplayer with a friend for a while now, and we've experienced a major bug when it comes to the mine.
So I went down one day by myself and surprisingly reached the end of the mine. I found the skeleton key and now ony my (and only my) questlog it says to figure out the purpose of the skeleton key.
My friend didn't get an update of his questlog though, it still says "Reach the bottom of the mine", although he has the skeleton key in his wallet, too. He already went down there by himself to see if he could trigger the quest to finish, but nothing.

It's pretty annoying because he hasn't played the game before, whereas I completed the story once in single player already. Is there a way to fix this? I'd really like him to get the new quest, too.