Sounds like your schema is expecting it to be a User object. If friendName has the objectId of the user, you can create the object and the SDK will convert it to a pointer
new Parse.User({objectId: friendName});
If friendName is the username, you'd have to first query for the User and then set it. Something like this:
new Parse.Query("_User").equalTo("username", friendName).first().then(function(user) {
requestFriend.set("username", user);
return requestFriend.save();
});