It won't work if he insists on processing all the files each time the report is requested, but I doubt that anything will. I assumed, perhaps incorrectly, that collecting the data by customer was a background process.

I figured that with all the data in the database, making the reports would be easy and fast, assuming that you indexed the table properly. It's possible this doesn't scale either, but the test program can easily be tweaked to tell whether it will.