I think you have to find out what was being signed for to really answer this question. It could be that permission was needed (a liability waiver?) to change painkilling medication or go on another medication to prevent blood clots for example, but again this is only speculation.

But from what you have described this sounds like a reaction to the pain medication i.e. a reaction to something like dihydrocodeine.

http://www.netdoctor.co.uk/medicines/100000744.html

there could also another possible cause of postoperative confusion, especially after orthopedic surgery.

http://mpatkin.org/surgery_clinical/post_op_confusion.htm

I hope that your brother makes a quick recovery.