It's true though. If the apocalypse comes and all hell breaks lose, you can't really trust anyone you're barely acquainted with, much less strangers. You have to be vigilant at all times and not let your guard down, else you risk yourself getting killed or worse. I mean, look at Black Friday in the US. A lot of people will indiscriminately loot stores like madmen, and that wasn't even a life threatening situation, just pure greed showing.
So yea, in this case crazy girl is right in a way. Better to move in smaller groups if you don't have the means to keep a large group in check.