I understand your issue very well. In my huge family it can be crazy too.
My ex sister in law was pregnant and my sister was pregnant with her only child that lived through the 5th month of pregnancy. My sister had decided on a name, such as Maria and the middle of Dawn. She had lost 3 babies, 1 was stillborn at 8 1/2 months (boy) and twins (boys) around 5 months due to her small size, they sort of fell out because her body couldn't support the extra weight. She had these names picked out for her very first baby if it had been a girl. Her 4th pregnancy produced a bouncing baby girl and my sister in law had had her baby girl a few weeks before and named her something and used Dawn as her middle name.
My sister was devastated and cried and cried and cried. She put on a good facade in front of everyone but she was so hurt. She used my middle name as her daughter's middle name in the end and it's very pretty and a great honor for me.
Can I suggest that you do go to her and sit down with her and tell her you were surprised by her baby name choice and that you hope that you can both name your children what you want.
I named my daughter 2 names I liked. My mother ragged on me over and over that those names were taken and that there were already several people who had been named that. I was just using old used up names.
But "I" wanted to name my child what I wanted. So I did.My ex and I did what we wanted and didn't worry about what others thought. But, if my friend had told me she was planning on having a child in the future and was choosing the same names I'd picked out I would have told her that was the name I'd picked out. Some time in the future she will hopefully get pregnant and have a girl and the names that are popular in that time might be something she likes better or she might want to use the same name. She could tell her girl she's named after yours or whomever she is named after.
Think about it like this. Kaylee. How many kids do you know with that name? Adults, teens, kids, but hardly any babies get that name nowadays. It's not "the" popular name now. I honestly know about 15 people with this name.
Names are special and personal choices.
Let's ask this question. Why this name? Is it pretty? Is it special to you? Your grandmother's name joined with your mother in laws maiden or middle name? Your favorite cousin's name? Why this particular name?
If you have a personal connection to the name you can tell your friend that you don't care if she names her future daughter the same exact thing but you want to let her know why you picked it way back when you and hubby decided on it.
IF it's not that personal why not toss around the idea of a different name? If she's a good friend let her have her name, if she really really wants it and it's not something she is thinking but might change her name in 6 months, then see how it goes.