I Have A “kind Of” Computing Ethics Question. Well More The Ethics Of A Prank.?

I was wondering if it is a little mean of me to go ahead with a practical joke I have been planning on that involves computers. Basically I have written a programme, which in essence hides a key logger. The key logger shouldn’t be blocked by anti-spyware or any firewalls as it is embedded in the core functionality of another programme that utilises the same client, it is a mini-Instant Messaging Programme that goes through a client on an old computer which I have modified to run as a server.
My plan is to give the programme to a friend, knowing that we’re friends he should trust the programme, which will allow it to operate through his firewall. (My version of the programme receives and stores the key log.)
Once on his system, the programme is not malicious in the sense that it will not harm his machine in any way, but to gives his information to me. Which is where the prank part comes in; but as I finished writing the code, I thought it might be a little mean to do such a thing to a friend. I don’t have a clue what sort of data I would retrieve, but the again it could be funny… (to me).
Should I? Does anyone have any thoughts on this?