If I was her I would want to be called the female name, and you should always be a nice person and treat others like you would want to be treated, I for one would think.
Actually, icelancer does get to decide. We all have to choose our nouns and pronouns, perhaps a relic of English lacking a neutral pronoun, but choose we must.
I think you'll find transgenders, et al, having to live the lives they do, have developed a far more nuanced take on public perception than fits with the role most of their self-styled allies have assumed on their behalf.
One of my first patients was an HIV positive stripper who presented with shingles. During the history, I noted the off-label estrogen and the B cup and asked "Darren, what pronoun would prefer?" "He is fine, but whatever makes you comfortable" he said.
I will always use "her" in reference to Pvt. Manning, but for historical accuracy, I try to split it between Bradley/Chelsea since it is usually more clear to the person I am talking to when doing so. I mean no disrespect.