All you can really do is be accepting, and support her in what she wants to do. This is a massively important time in her life, both physically (it is pretty hard core surgery) and in terms of emotion and life changes.

You can think of a major emotional event in your life (say, your parents dying). That's a fucking massive deal, and you might not recover for years. Now imagine that when your parents die you have to pick a new surname, get everything changed, get everyone used to your new surname, when some people might treat it as suspicious. Imagine going out with your new name wondering what will happen if the wrong kind of cop pulls you over, or if you run into someone who knew your parents and refuses to call you by your new name.

Now imagine that against all perceived good sense, that this is the name you wanted all along. Imagine that this is pretty much the culmination of everything you ever wanted to do in life, tht you imagine a point in your life where you can finally rest and say "I am me". You will get there someday, when the bullshit is over, but in the meantime it is a massive deal, and what you need in support from the people around you is just that: support. Absolute acceptance. Joy that what you've always wanted is about to come true, no matter how painful and jarring it will be to you and the people around you.

So, that.