You said all. He asked for her to stay with him forever, to keep him with her forever. And Nandini is the kind of girl who doesn't give up on the people she cares, and Manik is precious to her, the fireflies are a testament to that. It is going to be Nandini's turn to fulfil ALL the promises Manik made.
Manik might maintain a distance or blurt the truth to Dhruv, my hopes on the former one, but Nandini won't give up on him. She's going to pursue him, and get him back.