This is not related to Caylus or that player. I've seen that happen as well. I'm trying to think what makes it happen. It's possible the player just quit the game. I would advise trying a group abandon, and if someone refuses that, just wait for it to time out on its own, because I don't think there's anything you can do about it.
If you do want to report it as a bug, first, this isn't the place to do it. (Look up, where it says not to report bugs here.) And second, it's not related to Caylus so it would need reported under site-wide. (You might try to do a search for it first, since often the same thing gets reported over and over as a new report.)