I am a lawyer, but not a French one. I'm guessing that on this issue the laws of the US and France are fairly similar. A contestant certainly could be charged if the contestant purposely injured another contestant or engaged in grossly reckless behavior that injured another contestant. But if a crash between contestants occurs in the normal course of racing there would be no charges (or civil liability) because the contestants assume the risks naturally inherent in that activity. Locking tires would be a good example of something that wouldn't raise any eyebrows unless it was intentional.
She was facing away from the riders in a bid to get some camera time and while impeding the riders with her sign. Seems pretty negligent to me. This isn’t a case of an over excited fan accidently blocking the road while cheering her favorite rider on.