I would probably first try to recover the files from the drive with something like Recuva ( http://www.piriform.com/recuva ). Repairing the MBR is possible, but I've not had good success with doing so since it was probably pretty badly corrupted and is not nearly as easy as fixing the MBR of an OS drive.
And after recovering needed files I lecture the drive owner about the unreliability of external hard drives for backing up data that is not easily duplicated, as a review of this forum will show -- external HDDs are not very reliable backup, and are extremely poor for a single storage solution.
Yes, Recuva is freeware and will recover files after an accidental format or failure of the MBR. Download it and give it a try, it will not write anything to the drive so it will not overwrite files that are on it already.