OP still didn't answer.
If while waiting outside, the subject drew a weapon and pointed it at the cashier, what would you have done?
As far as the rest of the posts, there sure are a lot of posters whose only thought is saving their own skin, while leaving others to fend for themselves in the shark tank. I think that's real messed up. I'm not necessarily going to sacrifice myself for a stranger, but I'm certainly not going to abandon them if there's an option. Being a good witness, so the police can draw a good chalk line, well, that's definitely not me. If I see a potential threat, I'd assume a position that would prevent the threat from getting my six, and strike up a conversation with the cashier, and I'd be there until the threat was gone, or the threat engaged. I couldn't leave someone alone to get shot.