You should make clear to them that what you are telling them is confidential and that you are only telling them the information on the basis that they will keep it confidential. If you give them any documents, mark them as confidential. More importantly, ask yourself why the person won't sign and NDA. If they haven't given you a good reason (such as that they don't think the NDA you've given them is well drafted or its unreasonable) ask yourself whether it's really a good idea to entrust your idea to someone who has already indicated that they may not treat it as confidential, no matter how much you think they could assist?