She has a tattoo from before she was Muslim

Who has guided you to His Straight Path. I ask Allah to make you steadfast in following it. As for this tattoo which cannot be removed except through this kind of surgery, you do not have to do this, because it is too difficult and will cause pain and hurt. Allah says (interpretation of the meaning);

“… and [Allah] has not laid upon you in religion any hardship…” [al-Hajj 22:78]. Your regret and dislike of that, and your determination never to do it again is sufficient. Also, Islam wipes out what came before, so in this case leaving it as it is should not cause you any harm, in sha Allah. May Allah help you to do that which He loves and which pleases Him.