Oh bless her, does sound like she still missing her old mate maybe. I've still been having a look round for you as to what it could be and what could help.
http://www.theparrotclub.co.uk/topic/20-tips-to-combat-feather-plucking/
I also read about a spray like the ones you use to stop dogs chewing but can't find the link now :0/
I think the main thing I've read is that some birds just do it and as long as you feel you're doing all you can then the general consensus seems to be not to be too hard on yourself.
I had a Sun Conure that plucked and it was heart breaking, he looked like an eaten corn on the cob! Cured him by getting him a mate, but I know yours already has a friend.
I really hope you can find an answer.
Lisa x