I noticed on the Build-A-Bear Workshop Web site they sell hypo-allergenic stuffed animals. Ask the child's mother before you buy one if they would work for her daughter.
Ask your daughter if her friend is into any specific thing, like animals or crafts or art or games or movies or music. You can find something based on that/those themes.
You could also extend the child's happiness by giving her a "gift certificate" for a day out with your daughter when the child is better from the surgery.
Do check packaging for latex warnings - it can be in things you wouldn't have thought of right off the bat. (And remember NO balloons! They are latex! Mylar balloons [the silver foil ones] might be fine, but I don't know much about latex allergies so ask first.)
Also realize the mother is undergoing a lot of stress right now - everybody with a family member facing surgery or illness does, whether they show it or know it or not. So, you may want to do something/get something special for the mother, even if it is something small just to show you care and are thinking of her.
Additionally, if there are any siblings of this girl, you may want to get them something small, as well. They're not getting as much attention due to their sister's surgery, so will be glad to be remembered/noticed.
And, remember, besides the allergens, you can't go wrong - anything is better than just sitting in the hospital doing nothing!
