It would probably depend on whether Ember was actively stealing things and whether Bastian noticed. Also possibly whether they were both there with the Vestige and how willing they were to keep the peace for the Vestige's sake. I could absolutely see Ember pinching stuff just to annoy Bastian and Bastian barely holding his temper in check so as to not cause a scene for the Vestige.
Depends on who's party it is. If it's mine, for example, Bastian can go sit in the corner unless he behaves himself. Equally applicable if Ember is not there.