A Hasty Response
Some people need to learn to read a message properly before responding hastily. This guy was just trying to tell his girlfriend that he had “cheated off her” but she seems to have looked at the word “cheated” and assumed the worst. She also confessed her own infidelity in the bargain.
He clarified that he was talking about the Math test that they had taken earlier that day and she tried to cover up by saying her little sister had her phone. Obviously, he was not going to fall for that and broke up with a clever response. Seriously though, his brother?!