You'd be shocked how quickly people's thought processes can go out the window when somebody they know is in a direct threat of dying right in front of them. Even a single shared conversation can make a person feel attached to the degree where they wouldn't forgive themselves for not doing their all to help the other. 

It's easy to bowl a stranger over and run for cover, but what about the guy who just told you about how he can't wait for his kid's soccer game?

Of course, it all depends on what kind of relationship they manage to strike up beforehand. Also, whether anything in our hypothetical situation actually happens or not. I dunno, I kind of feel like placing my trust in amiable old Thomas.