Are you okay with mind-reading? Definitely, though scanning her mind thoroughly enough will bring up a seal blocking off certain memories (Namely canon events such as the existence of magical girls, Kyubey's contracts, Madoka's wish at the end of the anime, etc.).
Are you okay with short-term possession? Absolutely. Madoka's life is all about horror things.
Can he steal something? Same answer as possession! Do the things
If yes, what? Her kindness, memories of her family and friends, possibly even taking that seal off of her memories, if he feels like being that much of a jerk.