Let's start with the master of this trope, in my humble opinion: Leonidaslion. If your friend read The Suite Verse, she's probably already read these, but two that really fit what she's looking for:
-Set Fire to the Thousandth Bar (She wrote this for me. I asked for Protective and Possessive Evil Sam. There's also protective hellhounds that Sam sent to look after Dean. I love it.)
Dean doesn't remember much, but he knows who he's running from ...
http://archiveofourown.org/works/180227 (http://archiveofourown.org/works/180227)
-Indigo Blooms in the Kingdom of Hesperus
Just a routine morning in Hell, but Dean is starting to think that Sam wants more than he has been taking ...
http://archiveofourown.org/works/180206 (http://archiveofourown.org/works/180206)

To the End (Read the warnings on this! It's very dark!)
The world is ending, Sam has turned, and Dean stops running.
http://sometimesophie.livejournal.com/39469.html (http://sometimesophie.livejournal.com/39469.html)

Take My Hand
Possessive evil!Sam
http://sometimesophie.livejournal.com/50168.html (http://sometimesophie.livejournal.com/50168.html)

Carrion Comfort
in which Lucifer rises, and Dean falls
http://archiveofourown.org/works/5155 (http://archiveofourown.org/works/5155)

Climb Into My Arms With Blood On Your Clothes:
Dean rolls his eyes, gaze ending up on Bela. She's looking a little green around the gills and he can't really blame her. It reeks in here.
http://idlehands.ponderosa121.com/climb-into-my-arms-with-blood-on-your-clothes/ (http://idlehands.ponderosa121.com/climb-into-my-arms-with-blood-on-your-clothes/)

Yeah, I'm pretty sure she's already read all of leonidaslion's works, but I'm not sure about the others. (I've actually read all of them and loved them ;) )

Thanks for the help anyway :)

A Priori (http://memphis86.livejournal.com/8592.html) by memphis86 is dark!sam and reluctant!dean, sort of.

If it's okay that Dean's evil too then try Morgan's Broken Toys (http://www.sinful-desire.org/archive/viewseries.php?seriesid=355) series.