Witn modern airbag systems, it depends on the rate of decceleration, I think its at least 15G's or 15 times the force of gravity. I can tell you for sure tomorrow when I'm at work.
I would say that, at 30kph, and the fact she hit the car twice so it wasn't a dead stop, there wasn't enough of a decceleration to cause the airbags to deploy.

I can guarantee there would be no fault with the system, or the warning light would have been on, its really that simple.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i