you can do her death quest at any time. But you will need to revive her and pay the stars in Holy to get her. After she reaches lvl 100 you can take her to the place in the hanging garden area and rb her. I would suggest you try to burst her or lvl her up to lvl 100 first then do her death quest. The problem with doing the death quest to early when training pets is that if the pet leaves you because of low amity (if that happens) you will need to go back to holy and pay the same stars over again to revive the pet for a second time and then give it amity food so the pet does not leave you again. So my suggestion would be to lvl your pet to lvl 100 do her death quest and then rb her.