In the book (traitor legions), the favored of tzeench rule refer to ''all units in the formation can reroll any failed saving throws of 1''. In the case of the Tzaangor warherd formation, does the chaos sorcerer/exalted sorcerer benefit from this special rule?
Yes, if the Sorcerer/Exalted Sorcere was bought as part of that Formation and the formation met the criteria of the rule (i.e. if the formation contains the maximum number of units).
The Sorcerer/Exalted also gains Fleet as per the Formation special rules.
Unless otherwise specified, all units/models in a formation are subject to the special rules of that formation.
Source: Wrath of Magnus, p 150 - Tzaangor Warherd.
Also, from the Main Rulebook, p 118 - Formations text box:
Formations ...the Army List Entries that comprise a Formation are listed on it, along with any special rules that those units gain...