SS are a fucking disgrace.
She could try the NAS but ultimately she needs to get very heavy with the SW. I would suggest she needs to get her sister moved to better provision as well (she could ask the NAS to send her a copy of 'Communication' magazine full of adverts for decent adult provision). One excellent adult provision I know of is Coombe House run by the Priory. Your friend's sister may be too able (from the sounds of it) for the sort of services that Coombe House currently offer but I would suggest ringing the guy in charge of Coombe House and talking through things with him. Priory is a big organisation- he's a very helpful man.
Then land SS with the bill for decent adult provision.
In any dealings with SS you have to be ultra stroppy and ultra difficult. You have to tell them that crap provision is not acceptable, (you being 'one' one). That they will be responsible if something goes wrong and the sister is lost. Tell the SW she will be held personally responsible by the family. It also sounds as if she now needs 52 week provision whilst her father is ill and her mother is caring for him.
I'd really recommend ringing the Coombe house guy - search for it, and getting the lastest communication to research what else is out there.