Nope. I am 99% sure the stone is spawned through some script or something after the player chooses to save Steven or Boris. So there isn't any chance to take the gem, since it's never on her person.
After you meet up with Evelyn again, she won't drop a blood/star gem, not even when you didn't save Boris or Steven. Of this I am 100% sure, since I tested this.
This sounds like something that needs to be patched/addressed by Larian.
