As per the Fan theory your Girlfriend is referring to, yes, Quentyn is the sun which set in the East.
Mirri's prophecy was:
"When will he [Drogo] be as he was?" Dany demanded. "When the sun
rises in the west and sets in the east," said Mirri Maz Duur, "When
the seas go dry and the mountains blow in the wind like leaves. When
your womb quickens again, and you bear a living child. Then he will
return, and not before."
AGOT - Daenerys IX
So for Drogo to return:
- Sun has to rise in the West and set in the East.
- Seas have to go dry
- Mountains have to blow in the wind
- Daenerys has to give birth to a living child
The fan theory is, that the prophecy has been fulfilled for following reasons.
When the Sun rises in the West and sets in the East
Quentyn Martell belongs to House Martell whose sigil is Sun pierced by a spear. He was born in Dorne, Westeros and died in Meereen, Essos. So The Sun rose in the West and set in the East.
From the Citadel:
Consider that Quentyn Martell -- "the Sun's son", referring to the
Martell sun -- journeyed from the west to the east, where he died
When the Seas go dry
The Sea that went dry was the Dothraki sea. Daenerys noted that the grass of the Dothraki sea had gone dry.
From the Citadel:
In Daenerys's last chapter she notes that the Dothraki sea is going
dry and the grasses are dying
And also:
Dany set off through the tall grass at a brisk pace. The earth felt
warm between her toes. The grass was as tall as she was. It never
seemed so high when I was mounted on my silver, riding beside my
sun-and-stars at the head of his khalasar. As she walked, she tapped
her thigh with the pitmaster's whip. That, and the rags on her back,
were all she had taken from Meereen.
Though she walked through a green kingdom, it was not the deep rich
green of summer. Even here autumn made its presence felt, and winter
would not be far behind. The grass was paler than she remembered, a
wan and sickly green on the verge of going yellow. After that would
come brown. The grass was dying.
ADWD - Daenerys X
And:
The next morning she woke stiff and sore and aching, with ants
crawling on her arms and legs and face. When she realized what they
were, she kicked aside the stalks of dry brown grass that had
served as her bed and blanket and struggled to her feet.
ADWD - Daenerys X
And mountains blow in the wind like leaves
The mountains that blew like leaves are the tricky part.
From the Citadel:
Two of the pyramids of Meereen collapse in smoke and ash following
Viserion and Rhaegel rampaging through the city and that she
menstruates
But from a fan theory:
Now the most confusing aspect of the prophesy (my theory for which is
kinda tinfoily so bear with me). ‘The mountains blow in the wind
like leaves’ does not refer to the destruction of Meeren or any form
of volcanic eruption but to Ser Robert Strong. It is widely believed
that Ser Robert Strong is Gregor Clegane (aka the mountain)’s
reanimated corpse. It is however noted that Gregor's head was
shipped to Dorne. This decapitation would mean that the one “mountain”
has now become two “mountains”.
I'd like to add that since Mountain's body is in King's Landing and his head is in Dorne, One could say that of the two Mountains, one has blown like leaves to Dorne. Citadel however points to destruction of Pyramids.
An alternative view is the one suggested by Joao Duarte in the comments, If somehow the Wall is brought down, it could also be something akin to Mountains blowing in the wind like leaves.
OMG IF DANY CAN HAVE BABIES THEN DROGO WILL RETURN?
We must note what the Citadel says:
This might be a real prophecy, or it may simply be a sort of curse to
strike terror and despair into Daenerys's heart.
And also:
This of course leaves us the need for a living child, and then the
"return" of Drogo... though our speculation is that if she has a
child, it will be a son that she names Drogo, thereby fulfilling
Mirri's claim.
There is also speculation that Daenerys bleeds (Menstruate) for the first time in months when she is lost in the Dothraki sea, Which means she may have miscarried a child.
Just past midday she came upon a bush growing by the stream, its
twisted limbs covered with hard green berries. Dany squinted at
them suspiciously, then plucked one from a branch and nibbled at
it. Its flesh was tart and chewy, with a bitter aftertaste that
seemed familiar to her. "In the khalasar, they used berries like these
to flavor roasts," she decided. Saying it aloud made her more certain
of it. Her belly rumbled, and Dany found herself picking berries
with both hands and tossing them into her mouth.
ADWD - Daenerys X
And:
When she woke, gasping, her thighs were slick with blood.
For a moment she did not realize what it was. The world had just begun
to lighten, and the tall grass rustled softly in the wind. No, please,
let me sleep some more. I'm so tired. She tried to burrow back beneath
the pile of grass she had torn up when she went to sleep. Some of the
stalks felt wet. Had it rained again? She sat up, afraid that she had
soiled herself as she slept. When she brought her fingers to her face,
she could smell the blood on them. Am I dying? Then she saw the pale
crescent moon, floating high above the grass, and it came to her that
this was no more than her moon blood.
ADWD - Daenerys X
And
She rubbed her fingers through the dirt, and grabbed a handful of
grass to wipe between her legs. The dragon does not weep. She was
bleeding, but it was only woman's blood. The moon is still a
crescent, though. How can that be? She tried to remember the last time
she had bled. The last full moon? The one before? The one before
that? No, it cannot have been so long as that. "I am the blood of
the dragon," she told the grass, aloud.
ADWD - Daenerys X
She didn't bear a living child, she apparently miscarried. But that shows her womb can quicken.
