This is completely incorrect. Promotional cards, including staff promos may be used in sanctioned play. There is no difference between these cards and their ordinary counterparts other than the "STAFF" in gold lettering. I have used staff promos in major tournaments such as Regionals without an issue, and have even had my deck checked while using them and nobody has said anything. I am not sure what made your friend's league leader think they are not legal for sanctioned play. As long as the card is a staff promo version of a card that is legal for the event's format, it may be used. In the case of Oranguru, it would be legal for both Standard and Expanded play.