"Hey!" you say. "What are you doing sneaking around here?" He turns towards you and puts up his hands defensively, backing towards an exit. "Don't you kill ol' Jimb!" he pleads. "I'm just the lowest rung of the Defias Brotherhood, man! They told me to take out the owner, I can't say no!"
Attack Jimb. Go through with your job. Yell to warn Lou. You can go through with your job without attacking. Assist Jimb. Maybe the job wasn't so worthwhile. Tell Jimb you'll cover for him. If he doesn't want to kill Lou, he doesn't have to.