You didn't mention who is paying the bills. Does the daughter have a job and she pays for her own cell phone or land line?
If she were my daughter, I would sit her down and tell her that I have noticed that when she spends a lot of time isolated and talking to her friends, she becomes very disrespectful and has a bad attitude. Therefore, I suspect that the time she spends on the phone may be the cause of her unacceptable behavior.
Therefore, because the privilege of having a phone always at her disposal is causing her to become rude, disrespectful and obviously not a happy person, mom is going to need to take it away until she is able to handle having a phone without letting it lead her into bad behavior.
By "blaming" the phone availability for her bad attitude, you shift from "you are a bad person" to "you are being corrupted by a bad thing". I think it's important to keep the focus on behavior rather than character. It also forces her to prove you wrong, namely, she must prove that she can have a good attitude even while being on the phone.
We had to do exactly this when my twelve year old son started having a bad attitude while spending a lot of time on the computer with his friends. It has been about a year since we instituted a "bad attitude = no computer" policy and it has been what I would consider a 90% cure for his attitude. He still has lapses but he is much more civil and all it takes is a reminder and his bad attitude gets mended pronto.
It is unfortunate that this has been let go so long. An eighteen year old is almost an adult, at least in theory. There's a big difference between restricting privilege for a twelve year old, who is completely dependent on you, and an eighteen year old who has more experience using leverage of her own and who probably knows how to push her mom's buttons.
If the daughter is using a land line, it's obviously a lot harder to restrict her than if she had a cell phone that you could simply stop making payments on, or restrict in other ways. One way would be to set up a "white list", which would mean you could control who is allowed to call you. That wouldn't stop her from calling out, of course.
Another option is to get rid of the land line altogether and rely entirely on cell phones. A bit more expensive (unless you already have cell phones, which most people do these days). I have never looked into it but I imagine you could also have certain numbers restricted through your carrier (white listing or black listing).
However she does it, your friend needs to understand that it is going to get a lot harder before it gets easier, and she may not be successful in altering the behavior of a person who is legally an adult. The fact that this adult is living in her home, probably without paying rent, gives her some leverage but she needs to think carefully before trying to move anything with that lever.
Don't ever bluff with your kids. Don't threaten something unless you are prepared to follow through with that threat. Eventually they will call that bluff and then you either have to follow through or be proved a liar. Once they know you are only bluffing all your leverage flies out the window.
Now, all that having been said, not enough was said for me to guess whether it is the phone access that is the bulk of the problem (as it was with my son) or if there is a bigger problem between the girl and her family. If they can afford it, I don't think counseling is ever the wrong answer. Any relationship, be it parent-child or husband-wife, can gain benefit from a trained professional who knows how to spot patterns of behavior and what to do about them.
