In your early childhood, the bottom few vertebrae fuse together, but this can be prevented through exercises designed to flex and stretch the lower back. So gymnasts and contortionists who engaged in such activity as children likely posses the flexibility to put themselves in a position approaching this. But in general, no.
Actually, as long as those guys are holding her, this wouldn't be too difficult. Might leave her sore afterward, but she definitely wouldn't be able to hold such a position with her own strength for very long.