What is confusing me here is that the app needs to be written in C++.
That is usually heavy stuff. If that is the only language known, so be it. However, if the author is writing a Windows app, then C# and SQLExpress would work well, and on everything else, SQLite would work well for a DB engine. It may be a case where filemaker might even work, but that exits the realm of free DBs.
I need more info....
Max.