One foundation of reliable encryption is trust. The use of encryption between two or more people relies on you being sure that the message you sent is properly encrypted to and able to be decrypted solely by the intended recipient. When using a friend's GPG key, you must be sure that the key was created by and belongs solely to your friend. Otherwise, you may send mail that your friend cannot read (if they don't have the key you encrypted to), or worse, mail that some other party can read (if that party does have the key you encrypted to).