Continuing your investigation as Scott Shelby, you'll go talk to the father of one of the victims at his shop. Enter the shop and either walk around or talk to Hassan. It doesn't matter what tack you try to gain some information, he really doesn't want to talk about it. When you go to leave, you'll ask about some asthma medication since you're there anyway. Head back into the opposite corner of the shop to find the medicine on the shelf. While you're back there, another customer will walk in, but, not seeing anyone, he'll hold up Hassan at the register.
There are lots of options open to you. First of all, you'll need to pick an aisle to go down if you want to confront the robber. The only aisle that will work for physically taking him out is the far one on the street side of the store, as you can approach him from behind. The next aisle over has chips spilled on the floor, which will alert the thief and the other two aisles will let you out where he'll see you approaching.
For weapons, you can grab a frying pan out of the far aisle or a whiskey bottle in the first aisle that leads to the robber's back. When you go down the first aisle to knock out the bad guy, you'll knock over a box and you'll need to quickly catch it by hitting the quicktime button. On the easy difficulty you can just put the box back, otherwise you'll need to slowly unfold the movement to put it back quietly. Sneak up behind him and jerk the controller down when you see the quicktime instruction to brain him with your weapon of choice.
Alternatively, you can try and reason with the robber to get to give it up. In this case, go down one of the other aisles and walk toward him until he spots you. Hold and when he tells you to put your hands up, and keep them held down while you're talking to him. You want to stick to diplomatic choices when figuring out what to say, so stay away from the aggressive ones. After a couple rounds of conversation, you'll get the option to tell him to put the gun down. Choose that option, then keep talking him down until he leaves. The whole time you're talking to him, you're also getting closer to him, so if you make the wrong conversation choices, you'll have a chance to subdue him with a quicktime sequence. If everything goes wrong, you'll take a grazing shot before he takes off.
Your other option is to stay at the back of the store and let Hassan handle it. Unfortunately, he isn't willing to part with the few bucks in the register, so the robber will end up shooting him after giving him ample opportunity to change his mind.
When the robber is unconscious or gone, you'll be able to interact with Hassan before leaving the shop. In the case that he's still standing, he'll thank you by handing over a shoebox he received shortly after his son was abducted. Inside you'll find a green origami lizard. If Hassan has been shot, kneel down beside him and close his eyes after he passes away.