I din’t see it that way. Her knees buckled. She may or not have lost consciousness but she became unexpected dead weight to him.
Her knees buckled while she was still leaning on the post. He grabs her arm and then watch when his foot steps down off the curb. That’s when his legs buckled from the weight of her and THEN she collapses, and he takes her weight and keeps her from going to the pavement.
She didn’t go falling forward until he was surprised by stepping off the curb and buckled a bit.