The file your relative has sent you is a Family Tree Maker backup file.
Using Family Tree Maker
If you have a newer copy (2008 or later) of Family Tree Maker, you should be able to do a restore from backup.
In a post dated 14 Feb 2009 11:33PM GMT in this thread titled FTMB files on the Ancestry Message Boards, Russ Worthington said:
A FTMB is a Back Up file for Family Tree Maker version 2008 or 2009.
If you have either of those versions, File, Restore from Back Up will
open that file.
Later posts in the thread say that the *.ftmb extension is also used by FTM 2010 and FTM 2012.
Instructions on restoring from backup can be found on MacKiev's website:
Using other programs
Some other genealogy programs offer the ability to import a *.ftmb file directly.
Once you have imported the backup into a new program, you can follow the process in your new program to export to a new GEDCOM. The process should be similar to the instructions linked to in the previous answer.