The Lord of the Rings: The Rings of Power season 1 concluded shortly after Galadriel learned that Halbrand had actually been Sauron in disguise, but she didn't tell anyone. This was a surprise since the elf had spent much of the first season trying to convince her people that Sauron was still a threat. Though she didn't have specific proof, there is a good chance that if she told Elrond such a direct claim in Rings of Power, he would have believed her. However, for some reason, she kept the information to herself.
Galadriel and Halbrand had developed a friendship throughout their journey together in Rings of Power, to the point that Galadriel had no reservations about bringing her companion to Eregion. Halbrand helped Celebrimbor, with a great deal of charm, by providing information that worked out how to stop the fading Light of the Eldar. Of course, Sauron-in-disguise had been manipulating the elves to make something that he would later be able to control with the One Ring (though he hadn't made it yet). Thankfully, Galadriel put the pieces together before his deception went too far— but she said nothing, and the Three Rings were made.
Galadriel Kept Halbrand’s Secret To Ensure The Three Elven Rings Were Created
After all that Galadriel had gone through and the reputation she had developed for being overly obsessed with finding Sauron, it seemed strange that she wouldn't immediately alert Elrond to what was happening. Sauron's influence over Celebrimbor in Rings of Power could clearly have been for no good reason. Yet, she allowed the advice that "Halbrand" had given to move forward. This is because, Sauron or not, the Elven people still needed a solution to the fading Light of the Eldar so they could stay in Middle-earth. After all, Galadiel had vowed that she would not return to Valinor until she had eliminated Sauron.
If Galadriel had told Elrond the truth about Halbrand, he certainly would have told Celebrimbor. It's unlikely that Celebrimbor would have created anything that Sauron had told him to, whether necessary or not. So, instead of risking the truth in Rings of Power, Galadriel did her best to ensure their creations were stable enough to resist any influence Sauron might have had. In allowing the creation of the elven rings, Galadriel believed she could use their power and influence balance in Middle-earth.
Galadriel Was Ashamed Sauron Tricked Her
This was the practical reason that Galadriel would keep Halbrand's secret to herself, but it's likely there was an element of pride as well. Before the events of The Rings of Power, she had spent centuries hunting down Sauron to the detriment of her reputation. The fact that she had saved his life, become his friend, and potentially even developed a little bit of a crush might have just been too shameful for Galadriel to share immediately.
There could have also been a darker reason that Galadriel didn't want to confess the deception. The events of the Lord of the Rings proved that Galadriel wasn't above temptation, further reinforced by The Rings of Power. Much of her arc in this version of the story has revolved around the need to sometimes "touch the darkness," and her relationship with Halbrand has contributed to this. Halbrand offered her the power of ruling by his side– and a part of her wanted it. Shame had a significant role in holding her tongue, but she might not have been ready to close the door on opportunity.
Was Galadriel Ever Tricked By Sauron In Tolkien Canon?
The Halbrand plot twist was a surprise since this specific form of Sauron's never existed in J.R.R. Tolkien's works, such as The Silmarillion. However, in canon, Sauron really did take a deceptive form, known as Annatar, and trick Celebrimbor into making the Rings of Power. However, in Tolkien's version of events, Galadriel was one of the few elves in Middle-earth to distrust the fair-formed Annatar, and she urged Celebrimbor to send him away.
In The Silmarillion, Galadriel's mistrust of Annatar and his creations did have some influence on the creation of the Three Elven Rings, which were mainly outside the power of Sauron's One Ring. Therefore, though Galadriel and Sauron's complicated history is unique to The Rings of Power, it all returns to the same result. Still, this relationship and Galadriel's now even more personal vendetta against the new Dark Lord makes each character a little more interesting— so it may be a welcome change.