Friendship is not an equal yoke. Jesus Christ is our friend but we are not equal to Him, nor is He brought down to our level. In this relationship with your friend, you are the greater, in that He that is in you is greater than the world. As such, your primary concern is thus: has the Lord put us together? If so then you are to show her the Father in your relationship.
Further food for your consideration:
Atheism is not a lack of knowing God exists, it is a lifestyle whereby one lives as if God’s presence is of no consequence. In that sense, we all know God exists, but only believers live as if that is true. If you have been placed with her by the Lord then it is likely that she will recognize Christ/the Father as He appears in you by the example you walk out. I have unbelieving friends who seek me out for counsel because they recognize the gift of wisdom God has given me. I love them and they love me. And, as I am led by the Spirit, I am free to share my wisdom with those who are just and with those who are unjust equally. This requires mature understanding.
2 Corinthians 6:14-17
King James Version (KJV)
14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
15 And what concord (agreement or harmony) hath Christ with Belial (Satan)? or what part hath he that believeth with an infidel?
16 And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people.
17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you.
James 4:4
King James Version (KJV)
4 Ye adulterers and adulteresses, know ye not that the friendship of the world is enmity with God? whosoever therefore will be a friend of the world is the enemy of God.
Proverbs 29:27
King James Version (KJV)
27 An unjust man is an abomination to the just: and he that is upright in the way is abomination to the wicked.
Genesis 3:14-15
King James Version (KJV)
14 And the Lord God said unto the serpent (Satan), Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:
15 And I will put enmity between thee and the woman, and between thy seed (All nonelects) and her seed (All of God’s elect); it shall bruise thy head, and thou shalt bruise his heel.
Nonelects have no control over hating God's elects because Satan and his demons control all nonelects.
Example=finding fault with someone even though he didn't do anything wrong, this happen to my physical family.
Example 2=older sibling using loaded questions (logical fallacies) and taking sides with a bully.