Harry Potter has given me countless opportunities to talk to people about Christianity. I was proud to welcome my dear friend home after two years of conversations inspired by our mutual interest in Harry Potter.
If you can make your interest in Harry Potter do some good, that's your business. Are you going to use your mutual interest in tarot cards and ouiji boards to do some good too? Where does a friendship based on the occult lead? You can do what you like. Who am I to interfere? Just remember how you might plan to answer Jesus at your final judgment (friendly words intended).
Keep in mind: it could be your final gasp of clean air, among other things.