During her date with Mitchell on day 3, Cassie texts her friend Lizabeth. She wasn't really having any fun on her date with Mitchell, partly because he wanted to talk about the Others, which is something she didn't want to discuss. Therefore, she started texting her friend in the middle of the date, which Mitchell thought was rude.
This refers to a young adult sci fi novel called The 5th Wave, written by Rick Yancey in 2013.