Why don't you ask her why she wears it? Don't do so with a critical or negative attitude, of course, but simply because you care for her. Her reply might open up opportunities for you to share the true meaning of the cross.
I don't know how she'll reply, but I'm afraid many people today wear a cross or carry a religious medal simply because they hope it will ward off evil and bring them good luck. But nowhere does the Bible promise this -- and in fact it warns us against putting our faith in any object that we think will bring us good fortune. The Bible says, "This is what the Sovereign Lord says: I am against your magic charms with which you ensnare people" (Ezekiel 13:20).
Does this mean it's always wrong for someone to wear a cross? No, not at all -- not if they understand its meaning, and use it to remind themselves (and others) of Jesus' death on the cross for their sins. In Jesus' day, a cross was an object of cruelty and suffering; only the worst criminals were put to death on a cross. And yet Jesus willingly went to the cross to die as the final sacrifice for our sins.
Pray for your friend, that she'll realize the cross isn't just a piece of pretty jewelry or a good luck charm. May she see instead that the cross is the proof of God's love for her -- and may she respond by giving her life to Jesus Christ.