Unfortunately, kids know how to guilt parents.....this is a sad situation. It sounds like the parent doesn't know how to take charge, set boundaries, etc. so the child is.
My suggestion would be to pick your battles, but it sounds like the daughter keeps her busier than a one legged man in an ice storm.....
When my daughter (soon to be 25) was a teen, it was REALLY hard...we didn't have cell phones or Facebook, so I am sure it's really hard now. I picked my battles about her room. Lies were not tolerated or acceptable modes of behavior. If she lied, she would be given extra chores, loss of tv/computer time (we had plenty of computer games!!) and she wasn't allowed to go anywhere but straight home. Rules were rules.
if she's a single mom- will the dad take up some of the slack?
Your friend is needing strength in many ways - she needs to learn how to set boundaries and enforce rules. no means no. She needs to stand up for herself, stop with the guilt. She's doing the best she can - now she needs to stop the child from ruling the roost. Tough love works, it's not easy - but it works.
When my daughter was 13 - we were in Germany - she was having a truly rebellious time. Over the summer I sent her back to my sister's house in Las Vegas..not to get rid of her, but to show her we're not TRYING to be mean. We are her parents NOT her friends. And that's hard - you want to be your child's friend, especially as a teen, but YOU MUST BE THE PARENT - guide through the rough spots, showing your child how to maneuver through life. While she was gone, she realized that we weren't so bad and realized we were trying to prepare her for life on her own.
If the daughter steals, turn her over to the police - that will be the hardest thing EVER - but it might be the wake up call the girl needs.
Has she been tested for drugs? Alcohol? Has she been tested for psychological disorders that can be controlled with medication? I'm sorry - I feel kinda funky - as my daughter wasn't this bad.
I will say some prayers so that your GF can get the backbone and tough love she needs to handle her unruly daughter.