H-
I threw up a lot and compazine didn't work, but Zofran(generic name ondansetron) did. It is much stronger.
Is she throwing up literally right after the radiation? For me the smell of the machine and the stuff in my mouth made me feel sick. If that's the case, she could try putting an icy cold cloth on her forehead, or sipping icewater, smelling something minty, etc. If she's going to a place where they do chemotherapy too they most likely have an ice machine somewhere in the building, or there's always the cafeteria.
All of those "comfort measure" types of things can help.
Also, if she is dehydrated at all, that can cause nausea too.
Good luck
Melissa