Fiona was left alone in the inn room after Sirius disappeared from the scene.
Forget about betrayal or pressure tactics. The man refused to do anything that could even remotely upset Eren. He destroyed the array disk right away to prevent other clans from Last-Blood Bastian from ever embarking on that road.
One could say that Sirius was spooked by Eren for some reason after he met him in private. Their meeting had only taken place once just outside the city of White Raven after Fiona organized it for them. The meeting was brief and only intended to serve as a formality to establish deeper ties between the White Raven guild and Last-Blood Bastian.
Since the meeting took place between only Eren and Sirius, she wasn't aware of what had happened between the two. Only now did she realize that Eren had managed to intimidate even someone as bloodthirsty as Sirius using means not known to her.
