You will have a chance to rescue Primrose once you reach Stage Five. Here's how to do it.
After regaining control of your schedule in Stage 5, do nurse shifts with Dr. Twig until you see Primrose and Nursula.
This is the trigger that brings Primrose back to his lab.
Visit Primrose at his lab to see how he's been changed.
Talk to AIKO in virtual reality to discuss a plan to free Primrose from his new mindset. You will formulate a two part plan: create a synthesizer of Nursula's voice and then use it to lure the scientist back to his vacbed so you can gain access to his implants. If you plan on completing step five with Alex's help, meet with her on the upcoming Saturday and ask her for help with freeing Primrose.
While at work, look through your footage of Primrose and find the video involving Primrose and Nursula tending to a familiar face. Send it to AIKO once you've found it. She will have your synthesizer ready the next day.
Send Primrose the fake order from your computer. Once he's back in his lab, sneak in to the lab and face the security mechanisms holding him in place. They can be defeated one of a handful of ways:
If you have 18 or higher Smarts, you can handle it yourself.
If you have completed the drone infiltration mission and kept the implant, you can let AIKO control you and handle it herself.
If you have already freed Alex, you can use the USB device she gave you to unlock it.
You can attempt to force the machine open using brute strength. You will need 20 or higher Strength and 16 or higher Dexterity to succeed. If you lack either, you will get a Bad End.
If you do not meet the requirements, you can back out without penalty. You can tell AIKO to send the footage again when you are ready.
Meet AIKO and Primrose in VR when you are at home. You can try to convince Primrose to your side but will be unable to due to his implant's meddling. There are variations depending on if you have 18 or higher Smarts, have completed the Pawn's manor infiltration, and/or having a 20 or higher in either Dominance or Submission. The important thing is if you have or have not freed Dr. Toru from SISSI's asylum layer. If you have, move to step seven. If not, you'll have to do that now to reach step seven.
Watch the cutscene of AIKO, Primrose, and Toru talking and finish the quest. Primrose will be returned to his pre-Stage 5 self along with some new dialogue options when you visit him.
However, the truth is still out there to be discovered.