ouch, poor little thing! i've destroyed my shoulder/collar bones on multiple occassions as an adult, and the best thing to do is button up/zip up shirts. there are a LOT of styles of summer shirts that button up the back - or even little dresses that zip or button up the back, thus allowing you to not lift up her arm, would be best in terms of pain and healing. hope she's feeling better really soon!
edited to add: also, true halter tops would be great if it's warm enough - the kind that tie around the back of the neck - just have her step into the shirt/dress, pull it up to her arm pits, then tie it around her neck... she wouldn't really have to move her arms at all for that!