It might be anything from trying to strike him on the face to an angry push. Pushing someone that you think is trying to push you? Yes reasonable.
You have conceded that she used force, and that he was justified in using some degree of force to stop her. We don't have to answer the question of whether it was a strike or an angry push. All we need to know is that he was, at least at this specific moment, justified in using some level of force to stop her. You've indicated that a "push" was a reasonable level of force.
You are justified in using any level of force you reasonably believe necessary to stop a threat of harm. She attacked him in some way, he pushed her to the ground. She immediately rose to her feet: can you conclusively demonstrate that, at this specific moment in time, it was wholly unreasonable to believe she was rising to continue her attack?
If you can't conclusively prove she was harmless at this point, then, under use-of-force laws, the kick could (potentially) be justified as an attempt to stop her continued attack.
