It doesn't often occur in RL conversation that you need to use gendered pronouns about someone who's in front of you, though.
I was always taught the old-fashioned "Who's she? the cat's mother?" i.e if you're talking about someone who's there, use their name not an impersonal pronoun.
So I'd address Mary as Mary if that's what they said their name was, and I'd talk about Mary to my colleagues when referring to them. I wouldn't consciously not say "she/her" but I would probably just avoid having to, which isn't too hard. I certainly wouldn't call them by an opposing pronoun to the gender they preferred.
Tbh, I am all up for any Trans person to call themselves what they like, and behave as they like, and be referred to however they'd prefer, as long as they don't wish me to accept that their desire to live as a different "gender" is not the same as them therefore being a different biological sex.
Because, biology trumps feelings.
I don't care about gender one way or the other, except to be riled that it's a stick to be beaten with.
But this is misgendering, cis privilege, etc etc. Sigh.