Stoppingattwo, I think listening to your friend and being non-judgemental is the best way to help her.
I adopted a 5 year old, two years ago, and older adopted children are most definitely not easy to parent. They all have some form of attachment disorder - even those that appear 'normal'. Often they don't show their behaviours to others, and this can make it extremely difficult if you're the parent of that child. Others see an apparently normal child, and then begin to blame the parent.
My biggest piece of advice to you is to never, ever, tell her "all children do that" if she is venting over some behaviour her 4 yr old has displayed. This is not helpful, and usually adds to the frustration. Generally, it is true that all children do the behaviour - but not to the extent that a traumatised child does.
My own daughter is manipulative and controlling. She's also defiant, oppositional, treats me with contempt, lies constantly, is verbally abusive, and steals. And that's just the short list of her difficult behaviours. All are a result of her traumas - and are symptoms of attachment disorder. She gets scared of thinsg that others wouldn't even notice - and her reaction is extreme.
(On the good side, she is also very loving, caring, bright, generous... ) I love her to bits, but the stress she puts me through, the emotions she evokes in me, and the constant battle to understand and help her with her feelings is enormous.
Your friend will have known what she was letting herself in for - but the reality is far worse than anything she will have imagined. This doesn't mean she'll want to give up - but does mean she just needs someone to listen without judging. It also sounds like she needs support. SWs aren't there to remove a child from an adoptive placement unless that placement has irrevocably broken down. They are there to help - and I do advise her to ask for help. She can't do this alone.
Go to Adoption UK, if possible, get your friend to go there too. Use the message boards there - she will get the support she needs. There isn't a bad behaviour there that others won't have come across, and have advice on how to deal with it. Adopted children generally need therapeutic parenting, rather than simply parenting.
Do keep lending your ear to her though, and giving her a break by having her children. It sounds like you're doing your best, and being as supportive as you can. And she needs this. Well done for sticking with her! Most friends tend to leave once they realise just how difficult an adopted child can be...