Complex sentence: She informs him that Ellie is being prepped for surgery: to produce a vaccine for the infection, the Fireflies have to remove and examine Ellie's infected brain, killing her in the process.
Split into simpler sentences: She informs him that Ellie is being prepped for surgery: to produce a vaccine for the infection. However, she also reveals that in order for the surgery to be successful, the surgeons have to remove and examine Ellie's brain, essentially killing her in the process.