It wasn't hard to believe that you will be betrayed by someone close to yourself. If you are betrayed by someone you aren't close to or you don't know, it could hardly be called a betrayal. But that's also why betrayal stings, right? You never thought that this person who you like so much could stab you in the back. It was like saying I don't like you after you like them.
Dhvani had this take when she saw the face of Vivek who really was so surprised. Maybe he never thought that he would have to face this day. It also made sense how he was imprisoned. Only those who were very close to him know about his schedules and habits.
He should have known that when he first started interrogating people. The person who helped them capture him was the first person he had to be on guard against.