Question about Dumps in Oracle (REMAP_SCHEMA)
Even if not told, OldUser can be easily found as:

IMPDP System/Password@TNS 
dumpfile=ThefileName.dmp 
directory=dumpdir
sqlfile=ThefileName.sql
include=user


This does not import anything, just creates ThefileName.sql under dumpdir with the SQL statements used for creating OldUser, including the username as well of course. Using that information, they will know what to specify in REMAP_SCHEMA=OldUser:NewUser.