Mona was working for whoever was trying to kill Ali from the very beginning.
Whoever it was knew Mona was bullied by Ali, but worshiped her. A used Mona as the first piece in the game and had her sending A texts and messages. Mona was promised that Ali would disappear and that Mona would become the popular one and have everything that Ali had.
When Mona found Ali on the street, she realized that A’s plan to kill Ali had somehow fallen through. Because Mona was obsessed with Ali, she saved her, but wanted her to disappear so she could still get her end of the deal. Mona was keeping a secret from A and was possibly the one to kill Sarah Harvey in order to trick A into thinking Ali was really dead.
She had Ali’s clothes and could have easily put them on Sarah’s body.