From an ethical perspective (also a freelancer), I think it's my job to warn my clients when I think a particular idea won't work and help guide them to either abandon it or find a better idea. It is, after all, why they hired an expert -- presumably I know more about the specialty than they do so why wouldn't they expect that their money is paying for my honest advice?

If the insist on doing it the way they want, I am happy to admit that they may well know more about the business than me and do it anyway. But I'd rather have a reputation for steering my clients in good directions with both advice and direct technical help. That way people will be confident I'm not trying to take them for a ride. Plus better technical specs out of it and clarity around what needs to be done.