Because this already happened. You know that. [ He's to the point, maybe even a little smug. Is he talking about the fact that she turned back time, or the part where he knows this is a memory and she doesn't? ]

If we want to stop him, you... might wanna leave it to me. [ And then he looks around awkwardly. He's really doing this, but compared to death lasers, a older handgun is really not as daunting. ] Unless you remember what's supposed to happen? I'm kinda just along for the ride...

[ And then he gestures for her to hide out of sight, unless she's got a better idea. Far as he can tell, stopping this guy from shooting someone is the end goal. ]