cpp => c plus plus (c++)
This is basically just bitcoin core's sourcecode... it's not a wallet file, it does not hold private keys or any other personal information, it's just part of the sourcecode, the only thing you can do with it is compile it...
unless your friend has renamed his wallet.dat to wallet.cpp (but that would be a rather big coincidence). If you try to open the file with a text editor, and you can read the code, it's just sourcecode... If you see a bunch of gibberish characters, it might be a renamed wallet.