Is it possible to accept input from an end user via terminal, or some other application?
I want to create a prototype for a friend, but he'd need to enter his wireless network and password to complete the setup of the project.
Since you have mentioned that all that you want your friend to do is enter the WiFi SSID and password, I am gonna assume that its where his interaction with the device ends.
In this case, I would recommend using the arduino YUN, it cant get simpler than that.
The YUN is capable of creating an AP network, your friend can connect to it and then go to the arduino home page and then connect to his WiFi.
Another option would be to use a matrix keypad and LCD screen and have your friend manually enter the details