Is it permissible to bring a hairdresser into the home without knowing the extent of her dress's modesty, or with prior knowledge of her immodesty?

It is permissible to bring a hairstylist to your home. However, if she is immodestly dressed (mutabarrijah) and you can find another who is modestly dressed (muhajabah) and can perform the task, then you should choose the modestly dressed one. If the only one available is the immodestly dressed one, there is no harm, provided that the women advise her, enjoin good upon her, forbid evil, and teach her. If you are unable to do more than that, then you must condemn the evil in your heart. If you fulfill your duty of condemning her immodesty, then her immodesty will not harm you.