Reading a Situation is something you do, when it's your turn to talk in the general conversation. When the MC describes something and then asks 'what do you do?' you can talk about how you're reading the situation, keeping your eyes out, etc. Or you can describe how you're beating the crap out of some NPC because he pissed you off. But once you describe whatever it is, the MC is going to say something, and you don't get to interrupt them to change what it was you were doing, because you don't like what they said.
Reading a Situation is not a perception check, it is not automatic or passive; to do it, you have to do it.
It sounds to me like the PC was beating up the Chopper, not reading the situation. As the MC it's important that your moves make sense, and come out of the fiction; if this is the first we've even heard of this loyal NPC in this scene, having him knock out the Gunlugger out of nowhere might be too hard a move, it's hard to say. If on the other hand we know this NPC is there, and we know this NPC is loyal/easily-pissed-off/whatever, or you've already described how he's picked up a bottle with violence in his eyes, then it seems pretty straightforward to have him smash the Gunlugger in the head.
Assuming the Chopper is another PC, I think it is probably equally great to just tell the Gunlugger what's happening and see what he does about it -- if he decides to react to the NPC with violence, that's going to make the Chopper react, etc. etc.
But really, it depends on what you think makes sense as the MC -- if this guy knocking out the Gunlugger with a bottle makes sense as your move, then that's what you say. Just remember that after that, you still ask the player: what do you do?