Obito: Do you think doing that will stop this war...? You, friends of the traitor...
Obito, calls them 裏切り者の仲間たち (Uragirimono no nakamatachi) which translates to ''Friends of the traitor.'' It's definitely plural. Not ''friend'' but ''friends''.
I beileve he's referring to Kakashi as a ''traitor''. I mean, Kakashi seems to be the best candidate for it after causing the death of Rin and actually impaling Obito. If he wasn't adressing Sasuke at that moment I would have thought he meant him, but that doesn't seem possible.
EDIT: Aggregator mentioned that Oro might be the traitor Obito was talking about and I'm quite inclined to believe that.
Obito knows Kabuto is incapacitated, but there are four Edo Hokage's roaming the battlefield at the moment. I guess he put two and two together and came to the conclusion that Oro was responsible for it. He also knows that Sasuke is affiliated with Oro. So, Minato being zombified by him and Sasuke being affiliated with him justifies Obito's sarcastic use of the word ''friends''. Additionally, Oro had betrayed Akatsuki by trying to steal Itachi's Sharingan. It's quite old news but that incident does nothing but add to the reasons why Obito would call him a traitor.