I gotta agree with Single Six/old bear on this one. If I were on a plainclothes assignment or off-duty, and for whatever reason I was out at a club or anywhere and such an altercation ensued, I would avoid involvement unless absolutely necessary. I'd rather explain why I didn't get involved (given a story like this one) than be in the hospital or worse because my best intentions were misconstrued by on-duty, uniformed officers responding to the call. Now, if for some reason my CI or a person I was protecting/handling was in danger, well then it's my job to protect them, and that adds another element to the mix. We don't know why the officer was there to begin with.
boredom, while what you say sounds great, it wouldn't apply in this particular situation at all. The fight outside the club involved dozens of people, and the plainclothes was being overcome. From initial reports and accounts, he started to draw his weapon and likely wasn't even aware that uniformed units were on the scene. He was likely dressed in a manner as to fit in with the crowd, mostly "urban" wear.
And I think AaronG is on the money regarding the sheepdog comment.