It could be whoever found him/spoke with him?
Not in "bite-scene", unfortintly
We'd replayed session from the very beginning, forcing Astarion to follow and talk only with my friend (she found him first, and found boar then herself) - the scene still plays only with the host's character. And, as I said, with different player's characters on different patchs the situation was the same. It seems that it works in the same way with all the scenes which start when characters go to sleep and doesn't depend from player's in-game decisions.
It's not actually such a big problem, but i hope it still can be solved somehow.
