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, we ask you to advise your female friend and remind her of Allaah, The Most High, and about the bad result of this relationship in this life and in the Hereafter. You also have to remind her that by disobeying her parents she is being undutiful to them. There is no harm in guiding her to urge this man to propose to her guardian to become engaged to her and marry her as a means of solving this sinful relationship. If she repents, or if he marries her, all perfect praise be to Allaah, otherwise threaten to inform her parents about what she is doing; this might deter her. If still she does not take heed, then you can inform her mother even indirectly so that she will try with her husband to wisely solve the problem of their daughter.
Allaah knows best.