I'd try copying it to a new name: oldname.xls
(Don't hurt the original!)
Then try opening it in excel.
If that doesn't work, I'd open the file in NotePad to see if it gave any hints
to what kind of file it really is.
You may want to check this for more info:
http://filext.com/file-extension/IDX
And if that all fails, I'd ask that person what the heck it is and ask him/her
to save it in normal workbook format. (Verify the versions of excel so that
your version can open that other version's files.)