One is diffusion of responsibility or bystander effects, which I think she's hinting at. In a crowd an individual is less likely to take action. I think this has a lot to do with our social instincts. Being in a crowd of strangers is a new thing. We aren't built for it.
On our own, we make decisions for ourselves. That's easy. A group is supposed to be a family or troop or somesuch. These will usually have a leader. Someone responsible for deciding that this is a mugging, that we're on the victim's side and that we are going to join the fight. I'm not sure it has to be a literal leader, but there needs to be a pre-established decision making process and the individual needs to know where they stand in it.
A second issue is that this is not really a question of "willingness to help" in the intellectual sense. It's a question of instinct.
A third is that in a city (or in modern life in general) we are surrounded by other people's problems. The homeless person we step over, the domestic violence sounds we don't call the cops on, the volunteer help line where we don't volunteer at and the charity we don't contribute to.
This is complicated. I think it's more about living in cities, nations and the global village than it is about iphones.