Fictional characters have been dying for as long as fiction has existed. Teenagers and other sensitive readers/viewers/listeners have been moping and swooning over it equally long. And they get over it. There are such things as genuine mood disorders that require professional treatment, but this ain't one of 'em.

If someone doesn't get over it, they've got deeper problems that do need attention, but the majority of readers of this particular book will probably not fall into that category, and there's no need for a special emergency Harry Potter grief counseling service to take care of the overload.