I don't even think it needs to be something as complicated as "Mel confided in a trusted friend who then betrayed her or blabbed somewhere that Cover could hear." All of the talents have official company accounts on discord/twitter etc as well as, presumably, email. Management/staff have full access to these accounts, obviously. Mel strikes me as someone who might say, engage in a conversation on discord with a friend (Kotoka perhaps? or anybody outside the company) using her company account - they're already doing it anyways to set up collabs and shit. Management then has access to any of these conversations. She may just have been talking about shit that she shouldn't have been talking about, and it may have even been caught by management during some routine check or something like that. If she already had a history of talking about internal stuff with outside parties, then that would definitely be grounds for termination.