Picking a baby name can be an incredibly personal process. The names people choose can say a lot about their own lineage, culture, and linguistic preferences, so when you share that with someone else it's usually with they expectation that they won't steal it without your permission.
Naturally, there are many different kinds of conversations people can have about baby names. If it's a hypothetical aside about names you generally like, it's not bad form for another friend to pick one of the bevy. However, if you confide in your pregnant best friend about the name you picked to honor your mother, that's a completely different animal.
In a recent post on the Am I The Asshole subReddit, a woman opened up about how she wants to cut off her (former) BFF after her baby name was stolen.
Howcanichangehermind fully laid out how her BFF stole her baby name, and why it means so much to her in the first place.