I would say that your brother should definately take the child to the ER!

From what I've been reading even with drinking lots of water there is still a very high sodium content in the urine that the kidneys have a very hard time deailng with. Also it appears that drinking large amounts of water and urinating like that can cause electrolytes to be imbalanced which can cause even more problems.

http://www.ncbi.nlm.nih.gov/pubmed/15516309

That's the closest thing I could find on authoritative information. Most things I ran across were just news headlines or web forums which I would doubt the accuracy of the info.

I hope the child will be ok and will not have any problems. Please keep us updated.