User blog comment:Damac1214/Update: Confirmed Fake: Orientation DLC supposedly leaked for Black Ops 2/@comment-74.95.164.37-20130220182700/@comment-6097632-20130221033806

It's happened to me too. If you delete him as a friend then add him again in an hour or so it should work. If that doesn't work try chatting in another party for a while then invite him or have another one of your friends invite him to the party