Al-Salam Alaikum Sister,
Adoption in Islam is forbidden, As Allah (SWT) states in the Quran that previously adopted people should be called after their fathers. Prophet Muhammad (pbuh) used to have a servant with the name Zaid who was called Zaid Bin Muhammad. Allah (SWT) asked the prophet to give up that adoption and call him after his true father, so he was called Zaid Bin Harithah.
The prophet Muhammad (pbuh) wanted to honor Zaid by getting him to marry his cousin Zainab bint Jahsh who was a noble lady from Hashim tribe. The marriage was not successful because Zainab felt that Zaid was not qualified enough to get married to someone noble like her. Zaid divorce Zainab after all.
It was the costume of people in the pre-Islamic era to not to get married to their adopted spouses. Because Allah not only wanted to cancel adoption but He also established new Islamic rules that contradict the pre-Islamic ignorance period. Allah made the Prophet Muhammad to get married to Zainab to establish that a man not only cannot adopt but to get married to his previously adopted son.
By this adoption in Islam is forever cancelled. Though we can still take care of orphaned children and support them. The prophet (pbuh) said I am and the orphan supporter will be so close to each other in the day of Judgment. But we call the orphans by their fathers' names and we make them that they are our brothers and sisters and we treat them well. The orphans are not allowed to inherit out of the supporters’ wealth, but they can be given some share of it, which should be less than the third.
As of marriage issue, it is a sunnah strongly recommended by prophet Muhammad (pbuh). The benefits of marriage are enormous. For a woman it gives her the emotional resort that she needs, her little secured kingdom where she feels safe and valued. She finds a partner to protect and find a shoulder to lean her head on. Marriage is a decent and respected form of relationship that preserves a woman her dignity and honor. Where else a female can find love and to give love to.
Children also are one big issue. A big value of Muslim woman is being a mother. In Islam a mother is regarded of a high status where paradise is under her feet. When the prophet was asked who is the one that best deserve my good companionship he said: your mother and when asked who should be next he repeated your mother for three times and then said it is your father. The references in Islam in that regard are enormous.
Please sister, advise your friend to find herself a decent Muslim husband who will value her, get married and to have children of her own. When age develops and she is alone she will change the way she thinks and it will be too late for her and regret by that time will not of much value.
You have an obligation by the friendship to tell your friend the right advice and it is her decision at the end, but you have to be sincere to her any way.
May Allah guide your friend to the right decision and guide you as well to provide her with the best advice.
