.bkf is a Windows Backup file:
http://www.fileinfo.com/extension/bkf
That version can only be read by Windows XP (Vista and Win7 use a different format and I don't think it reads the old ones - not sure).
What exactly do you mean by "They have it backed up, but it appears that the data is corrupted"? Is the backup corrupt, or is the data that was backed up corrupt? Was the backup stored on the same drive that crashed or somewhere separate as it should have been?
They SHOULD have multiple backups. If the last backup is corrupt, you need to try an older backup. Backups are archives (like zip files) you usually can't recover or repair a corrupt backup file - it's likely all gibberish.
If the only source of backups is the crashed hard drive, then you'll have to run some data recovery apps on the drive to try and copy their data off it. I would recommend buying Spinrite and running a Level 2 scan on the drive, then see if you can copy any files from it by connecting it via a USB adapter to another PC.
If you still have trouble copying files you may need to run Scandisk.
Spinrite recovers bad sectors, but doesn't touch the file system, which may have errors in the volume table of contents or the partitioning tables. Run scandisk and set it to "Automatically fix file errors" and try copying the files again.
If the drive is in real bad shape you may only get a few files at a time before the drive disconnects and you may have to power it off and on to keep trying (which is why I recommended a USB interface and not connecting it internally).
I talk a lot about Spinrite and recovering data from a crashed drive in this thread:
http://pcabusers.org/forums/showthread.php?t=50984
Hope this helps.
PS: Tell your friend he needs TWO backup solutions from now on: something on-site like an external HDD and something off-site like Carbonite or Jungledisk.