Well if she's in the hospital already, that is good. This is scary and yes brain swelling doesn't happen immediately but you'd think that would have shown up already.
I had a similar experience with my daughter actually. She was about 18 months and fallen and hit her head. She took a nap and when she woke up she was very unbalanced, dizzy, and not making much sense (which is hard to tell with an 18 m/o anyway!). Anyhow, I took her to the ER and we did a CT, etc. and in the end it turned out to be a really severe double ear infection. It was just a coincidence that she had hit her head but the ear infection was the cause for her being unsteady on her feet.
I would think that if this was the case with your friend, it would also be easily found since they've most likely looked in her ears already, but you might make sure! I hope they find the cause!