Um.. I don't know how about telling the effing truth?!
I'm sorry but this can't tell him because Hera is absolute a BS cop out.
She could have told him in school and explained everything and even got his mom involved because she obviously had her number back then and would talk to her, between the three they could of come up with a plan. I don't like it, but I can forgive her for not doing it then because they were both kids.
However when he told her about Emy and how he felt and what it did to him and she kept quiet about it being her, that I can't forgive. She could have told him and spent the whole night explaining to him everything and convincing him not to say or act on anything because she has a plan. Hell knowing her plan he can help it. How much shit and pain and heart ache is caused in RL, in every movie, book, series because someone hides the truth and the hero nearly ruins it because they are trying to work out the truth. I mean unless she is worried that Hera has some magic ability or is spying on every conversation, that every area of his house it bugged and the second Emma says a single word Hera will know so she couldn't say anyting.... well if that's true, it's already game over, she has no chance and we may as well walk away.
I know it's always easier to see clearly from the outside looking in, but Emma has had 8 years or so to think about it and deal with the emotion and think it through logically.
All Emma has done is shown despite her "feeling bad" she actually doesn't give a fuck about the MC and how much pain she inflicts on him, she clearly doesn't trust him because if she did she could tell him and convince him, but she doesn't trust him, doesn't care how much pain she inflicts. I still can't understand how she can't trust him with the truth but is willing to trust him with her best friend. Makes me wonder if all her emotion and friendship isn't just an act and the robot Emma her true self. She was talking about the psychopath in her case... I think she was really talking about herself.