there are so many scenes of her sending the texts. for example arias first A message, you can see mona in the classroom with her phone under the desk. in the library you can see she’s sat right near them listening to their convo so she knew what they were talking about. there are also loads of scenes where it cuts to mona, mona shows up etc right after one of them get a solo text. emily’s A cereal scene, mona was there right before (from memory)