bitcoin-qt
http://bitcoin.org/en/download BTC.dat perhaps should be named wallet.dat and be in the default location, which on linux is ~/.bitcoin/wallet.dat
or import the private key if that is what the .txt is.
Import is commandline only I think and simple to do..
bitcoind importprivkey "5yourveryveryveryverylongprivatekeystring"
Bitcoin address is not the same as the private key though.. he needs to have backuped the wallet and perhaps did that from the GUI saved as a BTC.dat name that he would recognize.