When The Stranger and The Dark Wizard’s paths finally cross, it's confirmed that they know each other from before all of this. It’s well known that Saruman was the first wizard to arrive in Middle-earth. Then this line seemingly alluded to the fact that in Rings of Power, this magical being was also the first of the five wizards to land there:

Fans had assumed they'd figured out the identity of the mysterious Dark Wizard played by Ciarán Hinds, but The Lord of the Rings: The Rings of Power showrunners have now debunked one popular

So, if the Stranger is Gandalf, then that means the Dark Wizard is either Saruman, Radagast, or one of the two Blue Wizards. Considering the Dark Wizard certainly looks a lot like Saruman, and refers to Gandalf as "old friend," it seems pretty safe to conclude that he is indeed Saruman the White.