Well I may be slightly biased as a bit of a human pincushion myself, but if she wants it, then it's none of your damn business.
People should be allowed to do what they wish with their own bodies and not have to put up with the insecurities and ignorance of others, especially when the result of said actions has no impact on anyone but the person who wants the modification.
Unless you have genuine, researched beliefs that having the piercing will be a serious risk to her health or well being and do not feel she has enough knowledge about your concerns, butt out.
So let her do what she wants.