The first thing you need to find out is what version of NAV the customer is using.
Earlier versions (I think pre 4) only allowed data to be imported or exported via a Dataport object. This supported delimited file structures csv, tab e.t.c Later versions of the software also now have XMLPorts, which as the name suggests allows XML files to be imported or exported.
Both of these solutions would require development work inside of NAV as there are very few standard import/export objects of either Dataport or XMLPort. These are normally written by the NAV solution center that provides support for the system or occasionally some companies I have worked with do have internal staff with this knowledge.
Expanding further on this there is also capability to read and write directly to Excel spreadsheets, but this approach can be painfully slow as it uses the Excel COM Interop objects to achieve it.