I layed unfilled travertine in someone's house as part of my training course and we most definitely did not seal it first. Defeats the whole object of the exercise.
Enduro you can go one of two ways.... either do as your customer says and let him take the consequences or tell him to let you get on with it your way or find someone else if he thinks b&q know your job better than you.
Good luck with that one..... I hope you talk him round!