Anscheinend braucht man alle Austern, wenn eine Freundin gestorben ist.
If you kill a girl before you start going out with her, she does seem to come
back to life and continue appearing normally. I can't guarantee that this will
always happen, but it has worked for me before. It does seem to take some time
for her to reappear, or it may be necessary for you to save, reset, die, or get
busted before she reappears.
If you were going out with the girl and you killed her on a date, then you're
in a situation similar to if you were dumped. You seem to need all of the
oysters, and then you just go to where you first met the girl and she should be
back (after a few ingame hours). Of course, this only works for girls who have
locations to go to.
I have confirmed that this works for Katie, Michelle, Barbara, and Helena if
you have all of the oysters. And just like with being dumped, I have yet to
find a way to get Denise or Millie back after you've killed them.