seriously though, if youve been asked to board and skim a bathroom, which youve done, THEN youve been told that skimming the board wont hold the weight of tiles theyre planning to use, why is that your fault?
Politely explain that theyre correct, the skim wont hold the tiles and youll gladly re-do the shower area in cement board or whatever to make absolutely sure however seeing as you werent made aware of any different specifications to what you were given prior to work commencing then youll have to charge them appropriately as an ammendment to the original plan...
If they dont want to go down that route, just tile away on the skim but they'll be aware its their problem, not yours if things go tits up...