It's worth noting that in the raws, Miko just said: もう自由にしてあげて instead of the "It's time for you to let him be free." in the translation here.
The orignal Japanese text kept the intended recepient more ambiguous as this was a message that worked as directed to both Zen and the spirit of his mother. I personally would've translated it as "It's time to let go". In Zen's perspective, it would come off as Miko telling him to let go of his guilt. And from the spirit's perspective, it's Miko telling the spirit to fuck off and let her son live his life.
The translation makes it come off that Miko is speaking to the spirit directly here, which I suppose she is, but as far as Zen is concerned, there's only the two of them in the room. And this just makes it seem that Miko was talking to someone else in that room in his perspective.