@beautiful_soul
Holy prophet (s.a.w) definitely wore an Aqeeq stone , there is absolutely no problem for you or anyone else to wear ANY stone that they may desire .
u know where the problem arises ,
when u cross the line , and associate different sort of beliefs to the stone itself ,
that xyz birthstone is lucky for a certain horoscope , and xyz is good for money and luck , etc. etc
it is actually associations like these which would make the whole thing as shirk .
if she wants to wear it , explain it to her that that she can wear ANY stone she desires , even Aqeeq if her intention is to earn a sunnah reward from Allah .
Just do not let her get trapped into the whole birthstone/horoscope trap .
that which stone is GOOD for what and etc. , no stone is good for nothing they are just stones.
just treat it like what it is , a piece of jewellery , thats it .