paschabchochma; There are different views of Gehinnom,
described by chazal, none of them sound enticing at all.
To those who said Gehinnom is a good thing, NO, it is not!
That is why we say everyday in the Yehi Ratzon we should be
saved (spared) from the din of Gehinnom.
Ultimately though, it is a major chessed from HB”H allowing
those who have strayed from the path of truth and have become stained with impurity, to have a way of being purified,
if they have not done enough on this world to correct their ways, and cleanse themselves properly.
paschabchochma; I would try to focus on Gan Eden with this girl,
as obviously if she worries about gehinnom, she must certainly believe there is a Gan Eden for the righteous. You can ask her what she feels her versions of Gan Eden would entail, how she would feel if she ended up there, or would she wish she could end up there, Always focusing on the positive. You can also mention in navi those who were wicked & evil & managed to change their ways to the extreme, finding forgiveness and redemption.
Surely she doesnt consider herself worse than them.
Talk to her philosophically and hypothetically,
let it sink in, letting her come to her own conclusions.
As sadness and fear have a power, so does happiness and faith, and the power of joy outshines and blinds its rival tenfold.
Hatzlacha!