Your question really concerns the rules for direct and indirect objects, especially when the latter is placed in front of the former.
Take a simple example to begin with: Peter gave Joan the book.
(NOT Peter gave TO Joan the book.)
The book is the direct object (ask: gave what?)
It is customary to place the indirect object (Joan) (ask: to whom)
in front of the direct object (book) without a preposition.
That's the construction that most native English speakers generally use.
Another way of saying this is: Peter gave the book to Joan.
Here, when the direct object (book) precedes the indirect object (Joan)
the preposition to is required in front of Joan.
In your sentence, added confidence is the direct object (give what?).
Practitioners of the method is the indirect object. (to whom?)
Your native speaker is right:
Principles give practitioners of the method added confidence.
So when you place the indirect object (practitioners of the method)
behind the verb and in front of the direct object (added confidence),
you don't use a preposition in front of the indirect object.(practitioners of the method)
But you do use a preposition when you place the direct object in front of the indirect object:
Principles give added confidence to practitioners of the method.
There are numerous sites that give examples of this usage.
The following may be helpful.