unencrypting files that somehow got encrypted

From: mm (mm_at_mm.mm)
Date: 08/17/04 

Date: Tue, 17 Aug 2004 23:06:24 +1000

A staff member has some files (he created in the past) which he can no
longer access. The files somehow became encrypted. He is the owner and
creator of the files. The files are located in his private users folder so
I dont think anyone else could have encrypted it.

As the domain administrator and recovery agent I have tried unencrypting
the file without success. I have exported his certificate and then
imported into my certificate store, but still no success in unencrypting.

Can someone help?