Our app added functionality to allow users to import certain information from an excel spreadsheet (all users will have Office 2007 already).
The development machine
Just ask your devs to check all dll dependencies. This is a clear case of "it works on my machine" where you need to drag the devs to the machine where it is not working and point out that their documentation is lacking in the "dependencies department". If you have access to the source code or can run ILSpy on the assemblies you can propably track this down yourselve.
Safe bet is that yo just install Excel, but this is hitting the problem with a sledge hammer. Your devs SHOULD know what is really needed.....