|If you did not write anything to the external drive in the interim than it is not likely your boyfriend's files were overwritten.|
That being the case there are only two possibilities. First, they were never there or second, they are still there.
Did you actually verify ALL the files WERE there after copying them to the external?
What program is indicating the files were overwritten?
When viewing the external drive in Disk Management, is the total capacity shown there the same as expected?
Did you have more than one partition on the external drive?
Is there unallocated space on the external drive?