A few months ago I would have told you very gently that YABU.
I could never understand people being precious about names, etc....until my SIL did exactly the same thing (I am also ttc with fertility issues) and as I was cheering when they rang to tell me they had just had a DS, I was gutted when they told me they had named him X, which was MY name (quite unusual, and a name I had always loved)!
I actually felt bereft. As though there was no point in me having the name because I wasn't going to have the baby to go with it.
So YANBU, but there is nothing you can do here. Maybe she remembered the name, but not where she had heard it? Or once you hear a lovely name, it is hard to not use it, or perhaps her DH had aways loved it, etc etc. Whatever it is , I think you would be wise here to remember that when someone hurts you like this, it is easier to attribute the hurt to ignorance rather than malice.
I know that I was so upset because I was ttc.
And I actually couldn't sleep or stop feeling anxious until I had sorted it (soooo disappointed!) My poor DH finally understood and so we sat down and picked out another name for a DS. Now we have a lovely name and I am telling no fucker we are keeping it to ourselves this time
and I have in my sights the babies we are trying for.
I understand, but you have to let this go. Don't ruin her first emotional days by her worrying that you are pissed off with her, etc.
Good luck x