I must see them in person. Race car drivers don't count. Nor does it count if they're wearing the helmet for some other reason and briefly entered a car. It must seem like they're wearing it primarily to reduce the risk of injury in a car accident in normal driving operations.

If they're wearing a helmet because I honestly convinced them it was a good idea, that counts. If they're doing it because I paid them or otherwise influenced their decision in some way that isn't "them thinking it's a good idea", that doesn't count.