What is your obligation? I would personally say it's the tilers responsability to ensure the surface/substrate is suitably prepared to accept what you are fixing.
However, if the customer is that off your builders, and the builder contracted you to tile it as it stood then the onus is on him. As far as the customer goes. If you raised concerns before hand and was told to go ahead then i would hope you wrote into your quote "to tile onto wooden floor, floor to be prepared by 3rd part/builder" really depends what conversations you've had.
If the builder instructed you despite raised concerns, and you have it in writing you can breath a sigh of relief. If none of the above happened and you don't have anything in writing then i would say you you're obliged by your trade knowledge to ensure it was done correctly.
However, if the customer is that off your builders, and the builder contracted you to tile it as it stood then the onus is on him. As far as the customer goes. If you raised concerns before hand and was told to go ahead then i would hope you wrote into your quote "to tile onto wooden floor, floor to be prepared by 3rd part/builder" really depends what conversations you've had.
If the builder instructed you despite raised concerns, and you have it in writing you can breath a sigh of relief. If none of the above happened and you don't have anything in writing then i would say you you're obliged by your trade knowledge to ensure it was done correctly.