I guess your diary dictates your actions. If you can pull another job forward and not lose money this week then all is nearly fine. If not, then it's time to do your books etc. Either way the customer has to be told the honest truth. This situation is probably not caused by the customer, but the customer should have kept you informed of the progress or lack of it of this project. The plasterer probably said that the walls should be in that state to receive tiles. If you can rectify this situation by doing the work yourself or employing someone to do it then the chances are you will come out on top (financially). Good luck.