Dudley was attacked by a dementor in Chapter Two - A Peck of Owls in the story, Harry Potter and the Order of the Phoenix.
Yet he seems fine in later books.
How did he recover?
The Dursley's can't bring him to a muggle hospital and expect a muggle doctor to cure him of a mind assault by invisible magical creatures.
This was a magical attack and most (all?) other magical attacks and maladies require magical intervention. The dementor attack is unique because AFAIK it is the only magical attack where a person can recover with a muggle substance, chocolate. If there is a canon source that says wizards need other intervention besides chocolate, then that would indicate Dudley needed something else such as a visit by a magical nurse or doctor.
Did somebody from the magical world visit the Dursley's and cure him?
Please provide answers from canon sources. (e.g. - books, movies, interviews with author, etc...)