You proberly won't be able to do this, but this site may give you some ideas:
http://www.ex-parrot.com/~pete/upside-down-ternet.html
His situation was that his neighbours were using his wifi network, and instead of encrypting it he decided to have some fun, and used a transparent proxy that all non-authorized connections would go through, and it would do funny stuff like mirror or rotate all images on a website, or have a blur effect etc...
So if the right stuff is available to you, you can do some cool stuff. Be warned though that accessing someones network (even a friends) without their consent and playing around with it may not be legal. I don't know enough about the law but I would say the best thing to do is just tell your friend to encrypt the network.