You cannot save Millicent in Elden Ring. No matter what course of action you take, she dies in every version of her quest’s completion. She either dies by your hand or after taking out the needle that was quelling her scarlet rot.
You can do Millicent’s questline after completing most of Gowry’s questline. He’ll ask you to give the Unalloyed Golden Needle to Millicent to cure the Scarlet Rot.
Millicent is one of the few likeable NPCs in Elden Ring. I’m sure many players would have gone to great lengths to keep her alive. Unfortunately, her fate is sealed and she’ll die one way or another.
Even if you choose to help her, you’ll find her dead near the place where her summoning sign used to be. After fighting the sisters, she’ll tell you she wants to be alone for a few moments. She’ll then remove the needle letting the scarlet rot dissolve her body. When you reload your save, she’s gone.
Why did Millicent take out the Golden Needle keeping her alive?
Millicent decides to remove the Unalloyed Golden Needle that was keeping her alive because she refuses to accept the path Gowry sets her on. She doesn’t want to live as Gowry wants her to live. She doesn’t want to lose her identity and prefers to die as Millicent.
Should I help or betray Millicent?
You should definitely help Millicent if you want to get the Rotten Winged Sword Isignia talisman. Save her from her sisters, and you’ll get the talisman. You can use it to increase attack power with successive attacks by up to 13 percent.
If you decide to betray Millicent and kill her, you’ll get Millicent’s Prosthesis. You can use it to increase Dexterity and attack power with successive attacks by up to 11 percent.
If you help her and defeat her sisters, you won’t be able to get Millicent’s Prosthesis. She’ll drop the Unalloyed Gold Needle and the Rotten Winged Sword Isignia talisman instead. You need to start a new NG+ cycle to unlock Millicent’s Prosthesis.
Keep in mind that Millicent is immune to rot. However, she’s weak poison and you can easily stunlock her.
Conclusion
There is nothing you can do to prevent Millicent from dying in Elden Ring. If you don’t kill her, she’ll remove the needle quelling her scarlet rot which will kill her. She’ll die no matter what you do.