Always tell them there will be a chance of the plaster coming off. You can be 100 %sure that studded walls (plasterboard) will need some patch work done before you can tile.
Internally rendered walls (plaster onto brick) are more resilient to scraping off the tiles. However, if the internal render (plaster) is old, damp or had damp before, then you can guarantee that it will come off with the tiles.
A good way to develop yourself if you haven't done already, is to attend a 1 week plastering course. Perfect for what tilers need it for.
Discussions on all aspects of tiling & installing tiles in the tiling forum.
removing old tiles 
Linear Mode
