How to support my closeted trans fem friend?

So I have a friend who came out to me a little while ago as a trans fem ( they told me that they weren't sure about pronouns but they were starting estrogen, but even before they came out they said they wanted to be a girl so I think that's where it's leading to)

And I'm trying to figure out how to balance not outing them to other people but not giving them discomfort by referring to them as he in front of other friends

I made the mistake of saying they in front of a group of people and they were confused about why I was calling them that so I had to backtrack and act like I didn't mean to do it so I know referring to them neutrally doesn't help. I just worry at the same time about looking like I don't care but my friends feelings by misgendering . I know this sounds silly but this is my first friend who is boy moding
And not out to anyone else. So I've never had experience with this.

For trans girls who boy mode, does it hurt more to have your friends play along with it? Word is it more preferred to avoid accidental outting ? It just makes me feel like a bad friend IDK