Or just don't exit combat while a party member is unconscious and dying. Keep it turn based until all enemies are dead AND all party members are either stabilized or dead.
This is im affraid not enough ...
How about if you start conversation while Astarion stands in running water?
Companions can still move during conversations. Thus Astarion will just walk out of the water.
There's an edge case where he falls unconscious in the water during combat, is stabilized, and then dialogue immediately begins once combat is exited. But this set of events is pretty unlikely I feel.
