Eh, just don't tell her about it. XD
It is pretty nifty, though. I just like how incredibly easy it is to make posts and stuff with it. I also like how "replies" can done on your own Tumblelog with the reblog feature, which can potentially allow your post to spread exponentially. Basically, you make a post. If your friend wants to reply, he/she would reblog it and add their reply to it. Then, your friend's friends would be able to see that reply and possibly add their own two cents.
It's kind of a cool system, really. Plus, you can set it so that all of your posts get automatically thrown up onto a Twitter account, which can increase your, uh, "fanbase" even more.