Thanks Kate and Karin! At home right now, I have cockatiel seed that looks very similar to that mixture in the link. I'll offer her that to start, as well as a mix of different pellet brands, and I'm planning to try all different methods of offering fresh food until I find some things she likes.
My visit with her this morning went really well! Her owner brought her downstairs, away from her cage, and she was a lot more outgoing. She warmed up to me quite a bit (even though she's still a little standoffish), and actually puffed and let me give her cheek and head scritches. I was also very relieved to see that she isn't as emotionally damaged as I thought. She seems to like wandering around on the floor, exploring, and getting into trouble like a normal galah.
She went straight for my shoes, so it seems the sneaker dangly toy was a good choice! I can't wait to spoil her with lots of things to chew on.
I was also relieved to see that she does know how to fly. A noise spooked her, and she instantly flapped up into the air, gaining a surprising amount of lift for an overweight, out of shape, partially clipped puffball. She's not exactly graceful, but it does seem like she was allowed to fledge and knows the basics of flight control.
She's a really well-behaved bird. Doesn't nip or bite at all, barely makes a peep, doesn't make a fuss when she's put back in her cage. She's skittish, but even after she gets spooked, she settles down quickly and rolls with the punches. Honestly, she really doesn't seem to have any behavioral issues. I think I really lucked out in finding her!