Done in
72 hours mode.
-Can be missed if you decide to ignore her.-
First off you need to complete
The Facts in the case file (day 3) and then simply wait till the multiple cutscenes appear with the military cleaning up the mall and wait for the final one where Jessie turns into a zombie and killing a soldier.
Now head back to the security room where you escort all survivors and you will find Jessie turned. Take a picture of her and the trophy will unlock!
Note:
Do not kill her and then take a picture, this won't work! You can kill her afterwards.
Hope this helps.
