Tell her to watch Supernanny for the 2 and 5 year old. There are a lot of episodes about little kids. Supernanny also works with teens, but there aren't as many episodes about that.
Her teens need to be responsible for chores - that teaches responsibility. (And how to do things in the house! Yesterday there was a great post telling us about a nanny who didn't actually know how to wash dishes!)
I'm glad you added that she's not spanking the teens. There are parents who do that to teens and end up losing their kids. I've read here where one mom actually took a belt to her teen daughter, and now they are estranged. It is not worth "winning" a power struggle with a child just to lose the war, no matter how much someone wants to rule the roost and the people around them. And she lost the war, big time.
I do think watching Supernanny will help her formulate a plan.
Good luck to her!
Dawn