...and tell the client he is an expert on "that", but you want a expert on "this". If you search hard enough, you can surely find some related topic, that the expert is actually little bit knowledgeable. But that is the easy way out.
On the other hand, if the client actually considers the person an expert, it is possible there is another problem. The person may actually know things, but may not be interested in telling you that. S/he might be misguiding you on purpose. Or if s/he is not really an expert on the topic in hand, you may even get that expert to tell the client that s/he may not be the right expert for the job, and someone else might know better.
So you actually need to analyse your specific situation in depth before figuring out exact action needed.