TBH, I think this is your fault, and you're looking for someone else to blame to make yourself feel better. You let a 12-year-old drive a radio-controlled toy into traffic, which could have caused a life-threatening accident. It's not a nice thing to think about and it's natural to try to pass it off as someone else's doing.
It sounds as if the driver was also shaken up, and she thought your nephew ran the toy at her deliberately. I mean, at 12, if he had no obvious learning difficulties, I'm not surprised she thought he had run the toy into the road deliberately.
She could have chosen better words, but it's hard to say the right thing when you're on pure adrenaline because you've just avoided a really horrible situation. So, I don't think she acted badly, no.
As for you, I think you chalk this up to experience. You didn't think your nephew would act the way he did - he did - so next time you buy him a toy to play with of this nature, you need to choose a more appropriate location to play with him with it, and you need to keep a much closer eye on him.
It's really lucky all three of you lived to tell the tale, rattled as you all might be!
Did you see exactly what happened before the car went into the road? Did your nephew lose control or did he send it into the road without thinking about the consequences? Now the dust has settled, if you (or one of his parents) haven't already spoken to him calmly about the incident, it would be worth doing so.
After all, the worst kind of accident is one that no one learns from.