The above answer is is good as any I can give. Anyone who hatched regularly knows fertile doesn't mean they hatch. And is it possible for a fertilized chicken to lay an infertile egg? I would some yes.
If he wants to know if you gaurentee hatch, of course no one can. I think anyone that deals with eggs regularly understands that.
i agree, you can't guarantee the eggs, who knows if that person knows what hes doing, even if he does, accidents happen, you can tell him what you've had luck with, sometimes its a crap shoot selling on csl