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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, what should be looked at is whether or not the marital relationship exists between this woman and her husband (they are actually still married). If so, then she (the wife) is not permitted to go out of his home without his permission. If she goes out without a sound reason, then she is a disobedient wife and she is disobedient to her Lord; for more benefit, please refer to Fatwa 84630.
However, if the marital relationship does not exist between them, then her husband does not have any right on her. In this case, she is permitted to go out without his permission and she is not permitted to stay with him in seclusion in this home or allow him to look at her without her observing Hijab. For more benefit on a wife living with her previous husband, please refer to Fatwa 90016.
This woman should not often go out in the manner which you mentioned if there is no necessity in doing so. It should be mentioned that a mere separation between the husband and his wife does not mean divorce as clarified in Fatwa 82732.
After this, we have to look at her coming to your home. If you are not disturbed or harmed by her visiting you, then it is more appropriate to be patient with her, as this is considered as keeping a good relationship with the in-laws and being kind with them, which is religiously required. However, if you are harmed or troubled, or face any difficulty by her visits in the manner which you mentioned, then it is permissible for you to refuse her frequent visits. When advising her, you may use indirect ways so that you will not cause her great harm, and you may seek the help of whoever may influence her in this regard.
Allaah Knows best.