Firstly, do not attempt to cast out any demons if you are in doubt whether any such thing needs to be done. Whatever is not of faith is sin. Not only must you be certain of the fact, but it must be a fact confirmed by Scripture. As you wisely said, "I don't know if she is just walking in a sin lifestyle". If you try to cast out demons that are not there, or even do so without the unction of the Holy Spirit, you are going to look very stupid and likely ruin the testimony and witness of the Gospel that may come from your lips.
Secondly, do not be focused on casting out demon(s), but on the salvation of a lost and dying soul. It would be far worse to have her heart swept clean and put in order if it is only to be left unoccupied and be found later in far worse conditions than the first. What is required in this, in all Christianity, is an intercessory heart.
Which is a broken and contrite heart that is clothed with sympathy, compassion, commiseration, mercy, tenderness for others.
As mercy is defined in this way: compassionate or kindly forbearance shown toward a person; compassion, pity, or benevolence. In this case, referring to the eternal torment of lost souls who ever walk in slippery places ready to be cast down into hell without a moments notice. And the thievery of God's joy in rejoicing over saved souls whom Christ died for. How did you feel when you first found yourself worthy of the fiery hell for obstinate rebellion against a Holy and Loving God? Should we not also feel likewise for those who have not the sense to see themselves as God sees them?
As George Whitefield pleaded, "And however some of you may hate me without a cause, would to God every one in this congregation was as much concerned for himself, as at present (blessed be God) I feel myself concerned for him. O that my head were waters, O that mine eyes were a fountain of tears, that I might weep over an unconverted, graceless, wicked, and adulterous generation. Precious souls, for God's sake think what will become of you when ye die, if you die without being converted; if ye go hence without the wedding garment, God will strike you speechless, and ye shall be banished from his presence for ever and ever. I know ye cannot dwell with everlasting burnings; behold then I show you a way of escape; Jesus is the way, Jesus is the truth, the Lord Jesus Christ is the resurrection and the life. It is his Spirit must convert you, come to Christ, and ye shall have it; and may God for Christ's sake give it to you all, and convert you, that we may all meet, never to part again, in his heavenly kingdom; even so Lord Jesus, Amen and Amen."