I know Petrus has some issues with Rhea, although I don't fully understand them. I just know he's an a**hole.
A quote from wiki:
If you take too long to visit her (Rhea), after Tomb of Giants (exploring 2 or more areas and killing bosses), Petrus will kill her. She will drop 7 humanities along with a Pendant at her location in Undead Parish. (Update: Reah will be at the altar for the 2 areas and bosses ONLY.)
I already know this, even though I wasn't sure about the specifics. So I always warp to Rhea immediately upon rescuing her, so Petrus doesn't kill her. I usually buy all her stuff and she is then abducted and later found hollow in Archives.
This time, I wanted to keep her there alive, just for a change, so I didn't buy her Divine Blessing/s. Rhea was in Parish for a long time, but then I found a Pendant. To make sure, I killed Petrus and he dropped the Ivory Talisman, which is evidence enough for me, that he killed her, as he doesn't drop Ivory Talisman otherwise and in it's description, it is written that it is used by female memebers of Way of White. Petrus strikes me as an extremely ugly woman....
I wonder why, when I clearly prevented her murder it still happened.
Does the "(Update: Reah will be at the altar for the 2 areas and bosses ONLY.)" mean that no matter what I do, she will die?
Or does it mean that Petrus will kill her after this "time limit" has expired and I can prevent her death by killing Petrus before he does his evil deed?
PS: If you're wondering why I want to save her, it is for my evil genius plan to create a harem in Firelink Shrine...now only to come up with a way to get Priscilla and Gwynevere in there.