HELP???

My daughter is also 11. If I heard what you have heard, I would have no problem saying that particular girl would be no longer welcome to stay at our house, and I would also have no problem not allowing my daughter to go to theirs. Obviously there's a lack of supervision on their end of things. You might choose to talk to the principal of your school and ask to remain anonymous so that someone can be aware of what's going on and keep an eye on things, or perhaps even speak to this girl's parents.

But what happens with your family is your choice. It's not for anyone else to decide. So if you're not comfortable with this girl being a part of your daughter's life, that's completely up to you, and well within your rights as a parent.

Yes, these days very young girls and boys can be shockingly inappropriate in their language and their behavior.
You say it almost made you faint but you didn't step in. To allow these children over to the house is one thing - to allow them to do at your house what they do in their own is another thing entirely. The point of having them over is to have some control over the evening.
11 year olds visiting at your house should be told to go to bed at a reasonable hour. It's a sleepover and they like to stay up late but there's still a limit but you have to tell them what the limit is. You can nicely say to your daughter's friend - 'turn off the cell phone now, Susan - it's time to go to sleep." If Susan protests, tell Susan as nicely that you can't sleep until they're safely asleep and nicely ask if you can drive her home as she's clearly not tired yet and you are.
If it's mid-evening and she's trash talking on her cell phone, you can step in, sit down and say " let's play cards, girls. I'll deal. " If Susan the friend says she doesn't want to, Susan is free to ask you for a ride home. And you're free to say "all I hear is cell-phone talking in here, girls, it sound to me like you're bored if all you're doing is talking on the cell phone. Let's turn the cell phones off and save the minutes, girls."

If they were 5 and hitting each other, you'd step in. If they were 7 and the friends was throwing the cat out the window, you'd step in. If they were 9 and the friend was making prank calls to your neighbor or their teacher on the cell phone, you'd step in. If they're 11 and the friend is trash-talking on the cell phone to other friends, step in.
And as you felt faint, you could easily step in and truly say" I'm not feeling well, girls. I need some quiet. Let's turn the cell phone off." Or I'm not feeling well, let's call your parents Susan and have them come pick you up."

If you can't structure their time and don't feel comfortable taking charge of other people's children even when they're a guest in your house, then certainly you shouldn't allow her over. Even if her language and behavior were both perfect - there can still be moments when kids get together that need adult intervention. Some people are not comfortable intervening when it's not their own children and if that's how you feel, then you certainly shouldn't have to take on the extra responsibility that other people's children always are when in they're in your house.

The good news is these girls are 10 and 11. In my area, girls started very early and I resented it as I came from another place.
Think about a limit on telephone and sleeping time. Check out some books from American Girls series for your daughter and her friend.

Good luck!

The fact your daughter is trying to "help" the other girl recognize that her talk/behavior is likely to result in early motherhood is a positive thing. It sounds like she already recognizes the other girl is "extreme" in what she says and does.
You might want to repost your question in this group: community.greatschools.net/groups/35870