Maybe your client was wanting ubiquitous Excel-compatible export/import - not for the back end to actually be Excel? Making any table in your app exportable to at least CSV is pretty straightforward.
I think that is perfectly logical, the project was basically a series of command line tools. I tried to swap out excel for CSV or at least TSV, the client demanded .xls in .xls out. Believe me we tried every suggestion and that was as far as the client would negotiate. We got it to work and I died a little inside.
