How could she make such a big mistake and then go about her business as if it never happened?
Related vocabulary: go about something/doing something
idiomgo about your businessto do the things that you normally do:Workers have been going about their business and police say there have been no further incidents.Residents in the area put on their boots and went about their business despite heavy rains.
go about (one's) business
To continue doing one's usual tasks. Oh, don't pay any attention to me, just go about your business.Go about your business, people. I'll let you know when we can start the meeting.
go about your ˈbusiness
be busy with the things that you do every day: He looked out onto the street and watched the people going about their daily business.