Yeah I don't quite understand what you're asking either... if she is flying GRU-IAD-SFO and both flights are nonstop then it's an all UA itinerary. Perhaps she is flying IAD-IAH-SFO? In that case it could be CO for those legs.

Yes, you always clear customs at your first entry point in the USA so she will clear at IAD. She will go through immigration at the mid-field terminal and after clearing security will come out in the C gates where United flights depart from. If she is indeed continuing on Continental she'll have to head to whatever terminal they fly out of at IAD.