Her friend keeps company with men and commits sins, and she refuses to accept advice. What is to be done?
Question

I am a university student. I have a friend who does not pray regularly. She is stubborn and does not accept advice, and she listens only to songs. She has a bad friend whom she refuses to keep away from. She does not go home during vacations unless this friend also goes home. Through circumstances at the university she has gotten to know a group of young men, claiming that she is helping them. She corresponds with them and talks to them. When she goes out she has to adorn herself and put on perfume, even though she knows the ruling on that. We have tried to advise her but she refuses to take out advice. What can I do to help her?.
Praise be to Allah.Firstly: 
Studying in a mixed environment is haram, and it causes a great deal of mischief in society. What the sister says in her question is only a small part of the results of forbidden mixing. 
We advise everyone who wants to protect himself and not to fall into that which Allah has forbidden to keep away from these mixed places as much as possible, whether that is for study or for work, because of the things involved that go against sharee’ah and because of the evils to which they may lead. 
Secondly: 
What the questioner has mentioned about her friend is very unfortunate; we ask Allah to guide her and bring her back to the right path. Your duties towards her are to offer sincere advice and guidance, and to remind her of Allah and that death is real, and that this world does not last. 
Allah has created Paradise for those who obey Him, and He has created Hell for those who disobey Him. If she responds to the advice then praise be to Allah. If she insists on committing sin and following the path of the Shaytaan, then Allah says (interpretation of the meaning): 
“The duty of the Messenger [i.e. Our Messenger Muhammad whom We have sent to you, (O mankind)] is nothing but to convey (the Message)”
[al-Maa'idah 5:99] 
“And remind (by preaching the Qur’aan, O Muhammad), for verily, the reminding profits the believers”
[al-Dhaariyaat 51:55] 
“So remind them (O Muhammad) — you are only one who reminds.
22. You are not a dictator over them”
[al-Ghaashiyah 88:21-22] 
“O you who believe! Take care of your ownselves. If you follow the (right) guidance [and enjoin what is right (Islamic Monotheism and all that Islam orders one to do) and forbid what is wrong (polytheism, disbelief and all that Islam has forbidden)] no hurt can come to you from those who are in error”
[al-Maa'idah 5:105]
Try to look for righteous friends who can help you to adhere to the truth. Beware of sitting with bad companions, for Allah says (interpretation of the meaning): 
“And it has already been revealed to you in the Book (this Qur’aan) that when you hear the Verses of Allah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them. Surely, Allah will collect the hypocrites and disbelievers all together in Hell”
[al-Nisa’ 4:140]
And Allah is the Source of strength.