corrupt datafile

A new client has a serious corruption error.  During a hot backup (not RMAN) 
Thursday morning it was discovered that a datafile was corrupted on the disk.  
When the database is started up, the following error occurs:

"ORA-01122: database file 5 failed verification check
ORA-01110: data file 5:
'/ora2/app/oracle/admin/dbn/data/dbn_data_01.dbf'
ORA-01251: Unknown File Header Version read for file number 5"

When dbverify is run against this file, every block is indicated as corrupt, 
which isn't surprising.

We have a copy of the datafile from Wednesday's hot backup, but they don't have 
the archivelogs necessary to sync this datafile with the rest of the database.  
And we don't have the other datafiles from Wednesday's backup, so we can't just 
go back to the state the database was in then.

Any ideas on what they can do?  I can't think of anything (other than going 
back to an export they have from 6 months ago, which seems to be the only 
historical backup they have).

--Terry

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