Just tell her nicely before she finds out. I even think a text message would be ok-depending on your friendship.

Something like.... 'Hun I feel a bit awkward brining this up but remember ages ago when we were talking about names and I told you we like ***** for a girl, well you mentioned it the other day as now being on your list. I don't want to cause any trouble but DP and I really have our hearts set on that name if we have a girl and truthfully it's the only one we agree on, there are no other girls names in contention for us. Anyways I was just thinking about it and thought I'd pop you a text.... Xx '