They've noticed. They've all witnessed how Henry is with Charlotte and vice versa. They act differently around each other. They don't treat others with the same familiarity and comfortability that they have. Chenry's relationship from Everyone's POV.
She chose him. The boy she's claimed to have mentioned many times before, but he can't quite recall. He's pretty sure he would have remembered Charlotte mentioning some guy she's seeing. Based off 'Cave The Date.'