A solitary goose is likely to be very sad.
I don´t get the bit about her being in a barn. do you not have an area in your yard with a kennel for her where you can keep an eye on her? I know you said she was house-raised, but geese are very well insulated (not so much the sebastopols) in...
I´d get some niacin into her. It seems that niacin deficiency is one of the biggest problems. It doesn´t affect them all, just some. So many here on these threads have helped their geese by giving them niacin. You can also use Brewers Yeast as it has niacin in it. also, to get her to eat...