Have her watch, A Serbian Film, if she wants an idea of what a real living hell is. Then she'll be more great full for her own mediocre life.

Poor girl the christian practice is to use your own imagination against yourself. Not sure if this will help but when I was young I used to believe that if I stepped on a crack it would break my mothers back till I heard another superstition that canceled out all other superstitions. It only works if you believe in it ever since then I stopped believing in it and my fears went away. Religion is just like the placebo effect shes only scared of hell because she believes in it shes only scared of satan because she believes in it. Dreams are just your unconscious mind thinking about something your conscious mind can't figure out while your awake. Truth is so long as she is around those that gave her those dreams and those that incourage it she may never be able to rid herself of these dreams I would say take a private trip with her to help her enjoy life but these christians are like parasytes they'll embed something in her memory that won't allow for rational thought ex: hell

Is there any faith that is not scary? I thought Buddhism to be a strong option, but on closer inspection..............?

Does hell exist? No religion necessary except for the exceptionally weak, and catholics, who aren't really christian at all until they think they need it. Just be a good person. If it gets tight, forgive, but past that there is always the 'eye for an eye' first testament option.

Wow -- so sorry that your cousin was obviously disturbed and/or traumatized by the threats of Hell in Christianity. Fear-mongering is one of the things I hate about religions.

I think that showing compassion for her condition is a starting point- since you have a concern for her, it sounds as though you do have compassion and are willing to help her. Does she still subscribe to Christian dogma? Does she know your worldview? Are you agnostic? Maybe show her your worldview and explain why you think that way. Explain how religions can be manipulative by forcing frightening scenarios such as Hell upon children. I'm thinking that just showing concern over the long term will go a long way to helping her get out of that nightmarish mental state.

Awe. I take the medicine, Minipress for nightmares. It works well. Maybe a medicine for nightmares will help?

Ok, read the rest of your post...

There is probably more to the story of why she feels the guilt. You might want to just ask her "what's bothering her?" and that she can tell you without judgement or consequences to anyone.

Tell her to:
Stop Judging herself (her judgement of herself does not supersede God's will)
Ask for forgiveness from God and then she must forgive herself.
Sins are not damnable. They are guidelines to avoid consequences.
Temptation is not a sin.

These are just general suggestions, but her key is Christianity. So, maybe asking someone she trust and confide in with Christian speak can help.

In Spirituality, there is no Heaven or Hell. Just Love.

tell her what a terrible god it is if he existed and that he created the devil so its just blackmail. show her what a crock of shit religion is.

Martial arts or mediation can soothe the mind and make one into a calm warrior spirit
I remember distinctly when around 8.or 9 and a teacher brought a New Scientist magazine in and told us how the biblical flood was now thought scientifically possible and that scientists believe it could happen again. I was terrified for months.
Then he told us about spontaneous human combustion! Don't think I slept for 3 months!! Obviously I know now it's claptrap.

Teachers have a lot to answer for.

She needs to learn to control her dreams as I do, I have not had a nightmare in 45 years since I learned to do so. I believe the technique is call lucid dreaming.