Steve,
Check ceiling of the main lounge for signs of past water damage/cracks/leaks etc, plus take a look at the guttering & runaways on the roof.
Personally I would not touch this job with a pressure washer, better off using a floor scrubbing machine with bristle pad and lightly damp!
Make sure you get the customer to sign a waver before you start the job excluding you from any possible damage/responsibility etc, otherwise walk away!
keith