Yes - Magic Kingdom is DEFINITELY decorated for Halloween. It's very festive, actually. If you can, try searching for photos or videos online - it will give you a good idea of what it looks like.
The party is a lot of fun too. My mom and I went last year and it was so cool to see everyone dressed up and the parade is AWESOME. If your DF loves Halloween as much as my DH does, he will enjoy it.
Poly: Mar90, Jul91, Mar95, Dec98, Jul00 (future DH 1st trip!)
WL: Oct02 (Honeymoon), Dec03, Feb07, Mar09 (DS 1st trip!)
Other trips: Apr94,96 offsite, Aug99 Dolphin, May07 DL
Dec09 AKV Kidani
Apr10 Poly (20th WDW "Anniversary")