What should I do if that girl is killed by QJ?

Comfort her more. What she needs most now is someone to comfort her, protect her, let her slowly return to normal state of mind, be kind to her, spend more time with her and make her feel safe. This can't be recovered in a day or two! Tell her you still have me, I will protect you, don't always want to die, live well! In short, let her feel safe with you, and she will be very happy to see you!