Have a look in the folder that it is installed in. See what files are in there, in windows explorer look at the type column and see what types of files they are, just from the description of the type you might be able to pick the file.
Also if they have been using it for a while and there is a lot of data in it it will probably big quite a big file so sort on the file size by clicking on the column header.
The simplest outcome would be if it was a MS Access database but that might be too much to hope for.