Car break in-what's the right thing to do?

Tourist here from London. Yesterday witnessed this crime from start to finish along Van Ness, just south of House of Prime Rib. 2 guys turned up within 5 minutes of the owner parking her car and smashed the windows to grab her bag. I was watching from my balcony and didn't cry out because I was worried about my safety. Called 911 and two policemen turned up within fifteen minutes, and gave the owner a crime number for her insurance. I feel bad I did nothing to stop this. It was weird because people were just walking and cycling past with zero reaction. London is unsafe too but not like this.