All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, it should be noted that it is not permissible to accuse people of having committed obnoxious acts without evidence, as in principle a Muslim is innocent until proven guilty. Allaah Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.}[Quran 49:12] However, if it is proven that a specific person is in relationship with a non-Mahram (marriageable woman), or that he has done what displeases Allaah with her, then whoever knows this should repudiate this act from him. This woman could threaten him to take the matter to whoever will be able to prevent him from this act.
It is absolutely not permissible to allow a man to enter upon marriageable women without the presence of their Mahram (non-marriageable relative). The Prophet said: "Beware of entering upon marriageable women." A man from the Ansaar remarked: "O! Messenger of Allaah, what about an in-law? He replied: "He is death itself (i.e. his seclusion with women is as serious and dangerous as death)." [Al-Bukhari and Muslim]
The people who were tolerant about this matter have indeed committed a sin, especially the relatives, so one has to be careful. Besides, one should be careful of taking home maids, as there are many disadvantages in doing so. However, if there is really a need for a home maid, then the religious requirements should be met. For more benefit, please refer to Fatwa 83374.
Finally, it should be noted that if it is proven that that maid has a relationship with that man and she has not repented from doing so, then it is an obligation to dismiss her from work as she might corrupt other people in the household.
Allaah Knows best.