She will still give you the Pure Faintstone with 20 Faith, if you have already rescued and talked to Saint Urbain in the Nexus, as long as you don't exhaust her conversation.
I got it up until the "I plan to bequeath all of my possessions to Saint Urbain." part, will try after that as well some other time.
Tried this again. Defeated a lot of bosses after rescuing Urbain, but she didn't get to the "I plan to bequeath all of my possessions to Saint Urbain." part until I defeated Old Hero. When she finally did, I left the Nexus, returned, bumped my faith up to 20 and talked to her. Now she's only repeating "But Saint Urbain can hear the voice of god through it!", and no gifts. Defeated Storm King, and her conversation jumped to the next part, no gifts.
Did it once more, bumped faith up to 20 after her "I plan to bequeath all of my possessions to Saint Urbain." dialogue. This time without leaving the Nexus, but the outcome was the same, no gifts.
Once more, this time she started her "I plan to bequeath all of my possessions to Saint Urbain." dialogue after I killed Astraea. Ran away before she finished, leveled Faith to 20 and got the stones.
So you can get the stone, after rescuing Urbain, if you dont talk to her with Faith below 20.