In an interview with Variety, Noah talked about how he came out to her.
“I kept trying to do it in person with her, and it was too hard, so then I just FaceTimed her one day in the middle of a Party City and I was like, ‘Millie, I’m gay.’ And she was like 'Oh, Schnapper! You told me finally!’”
She hit him with the "I knew all along!"
Honestly, that's one of the best reactions you can get. I also love that it happened at Party City.