Yep, it's PTSD. Chloe came out of UC2 remarkably unhurt physically but considering one of the first ways she tries to talk Nate out of the Iram quest is to remind him that "things got more than a little dicey" "the last time we went halfway around the world searching for a lost city", I think watching two men she cared about almost and actually die had more effect on her than she lets on. I'm fascinated by the ways the awful things that happen in my canons might affect the characters after the main event's over (this is probably why my first fic for this fandom was about grief) and sometimes it gets into my headcanon, oops.

Charlie's a smart guy and I got the impression from UC3 that he kind of just goes with his friends' quirks (he never questions Drake being Nate's ancestor even though he's probably done the "Drake had no kids" research). When that quirk is Chloe's and it's getting her genuinely upset, he is absolutely going to be considerate of it.