Caveat: I haven't used Stadia myself, but have examined the developer documents, so this is a best guess.
Assuming you can cross-play between Stadia and Windows versions of the game, then as long as the Stadia server and the PC acting as host to the game have a good connection, then the Stadia player will appear to still be there, even if they lose their stream connection to Stadia ( beacause Stadia will maintain the session for several minutes waiting for the player to re-establish the stream connection ). Eventually, Stadia will terminate the session.
This may not be ideal, however, as the Stadia player's character will not be receiving input during a streaming outage, and so will remain idle. At least, since combat is turn-based, there is little chance that the character will die; but you may end up with the situation where it is the Stadia player character's turn in combat, and the game cannot progress until either the player regains control of their character, or Stadia gets bored and terminates the session.
Edit: Since the MP aspect of BG3 is under Larian's control, I would assume that they can add code to the Stadia version that recognises when a player has lost their streaming connection; this might allow them to offer choices to the game host about how to proceed. Maybe someone from Larian can answer whether this aspect has been considered yet.
Last edited by etonbears; 19/09/20 01:10 PM.