I'll try to explain how the whole contraption works.
The "user interface" is a few hundred automatically generated spreadsheets on Google Drive. About 40 people use it to enter data. These spreadsheets do some formula calculations that are mostly used for human verification, but they're also checked for consistency with the calculations in the reports (more on this later).
A Ruby script downloads each of these spreadsheets as csv using the Drive API, then copies each cell to its correct location in the "database" Excel file using WIN32OLE. The Excel OLE API is surprisingly well-documented (but not well-designed, unfortunately).
The "database" Excel file does some formula calculations. It has tens of sheets, and rows of the same number on every sheet represent an "object". The good news is that formulas don't go outside a single row. There are also some "summary" sheets that have formulas spanning pretty much all cells.
Reports (we need a separate one for each "object") are handled by another Ruby script, which simply copies values from the "database" into template Excel files. The most important calculations are done in formulas in the report files themselves. There are also a few summary reports featuring management-friendly graphs. These reports also do some calculations of their own.
At the end, the "application" is really just a bunch of Ruby scripts pushing numbers around, and it goes through great lengths to avoid doing any calculations in the Ruby code.
All that said and done, doing the application properly would include not just rewriting the Ruby code, but also implementing features from Google Drive (online concurrent editing, Excel-like interface that requires little training, block selection, copy/paste, etc) and Excel (Easy-to-edit reports, pretty graphs, etc).
So a rewrite will probably keep some of the current developer-unfriendly but user-friendly parts (Google Drive, Excel reports), but use a real database and do the calculations in code, not formulas.