Short answer: I tend to think of this more part of an erotic dream than anything directly connected with the dark spiritual world. (Of course, I could be wrong.) Certainly you are right in pointing her to Christ. The problem may simply vanish once she has repented and been baptized (Acts 2:38; John 3:5; Titus 3:5).
Yet her situation seems more complex than most non-Christians'. Now without knowing her and her situation, it’s impossible for me to be of much help. Yet if I were involved (shared in her and your culture), I would begin by asking her a few questions:
- Is it possible you got the idea of this incubus from a charismatic church? (I know that sounds funny, but we shouldn't underestimate the power of suggestion.)
- Have you been sexually abused? (As a child? Later? Was it a family member?)
- Do you know what the Bible teaches about powerful temptations / experiences? I would show her 1 Cor 10:13 — a promise that (in Christ) we will not be overpowered by any temptation. I would also show her Col 2:13-15 — whatever the powers of darkness are, in Christ we are freed from them. They have no power over us.
- Are you sexually pure? Are you living a celibate life? (Ignore this question, obviously, if she is married.)
- Are you open to professional counseling? I wouldn't seek just any counselor; make sure the professional is faith-based.
- Will you read the Word every day? Are you aware that the word will be written on your heart, and you will be protected, if you read and understand God’s word? I would show her Col 3:16 and Eph 5:18+. If she is not serious about learning, I would tell her there is nothing you can do for her. The power is in Christ!