Often violence plays a few key roles in making a nonviolence movement successful. One is that it gives the opposition an opportunity to look seriously at the nonviolent practitioners as a more legitimate and civil entity, which gives them much more power. For example, one of the reasons that Martin Luther King and other leaders of the Civil Rights movement were taken seriously is because of other civil rights organizers such as Malcolm X who were advocating violent means. For many people, Martin Luther King would seem a little crazy and radical were it not for these groups of violent people who were even more radical. In many ways, the violence movements helped convince whites to support the nonviolence movement because it seemed like a much better option.
This example, while valid, is not really what my grandmother's friend was talking about. She mentioned that nonviolence movements often provoke violence reactions from opposition. In many ways we can see examples of how this happens in a variety of movements. Gandhi in some ways provoked violence through taking the salt mine, and the violence done with the school children being beaten in Montgomery. All of these actions of violence were used by nonviolence movements to help gain empowerment and motivation among members who feel as though they are being wronged and have common suffering to share. Is this violence necessary for the nonviolence movement? I would say that it isn't but it does make it much easier for bystanders to make up their mind and empower sympathizers to join the movement. I think it is a lot more difficult for nonviolence social movements to function when the opposition is also using nonviolent tactics. It is certainly not necessary but a lot of movements in nonviolence may not have had the same success if otherwise.