I run electrum on my PC and it creates / saves a wallet file with the type 'text/plain'. If it is by any chance an electrum wallet, you could try to place it in the wallet directory (
http://docs.electrum.org/en/latest/faq.html#where-is-my-wallet-file-located) and then see if it opens up.
Usually in Bitcoin Core it creates a wallet.dat file and in the old multibit classic it created a .wallet file.
Multibit also used an encrypted format such as .wallet.aes or .aes:
https://multibit.org/help/hd0.1/files.html (It was a deterministic wallet) and blockchain.info used to use a plaintext JSON format.
So if it's neither of these, you should either ask your friend if he / she can recall the wallet that they've used. Also confirm if that backup is a paper backup or paper wallet, to restore on armory or any other wallet. If you're still stuck, try to back the file first and then create a copy of it and open it with a text editor to observe the contents, maybe you will be able to find how that content is related to a wallet that your friend used by searching online about it.