Which initial attack?
The grabbing of the glass was unjustified, but not an attack. The throwing of the liquid at her was unjustified (but provoked), and also not an attack. The initial "attack" was when she stood up and began striking him in the face.
She was getting up
The rest of your comment on this point is irrelevant. She was knocked done because she had attacked him, and she was getting up before he had a chance to back away. If he reasonably believed she was getting up to continue striking him, he was justified in putting her back on the ground.
You can at this point only speculate that she was trying to attack not really prove it.
Correct. Absolutely correct. I can't prove it; he can't prove it. But, if you look at the laws governing use of force, he doesn't have to conclusively proven her intention to continue the attack. All that has to be shown is that a reasonable person in his position would believe she continued to pose a threat, and he is justified in using a level of force a reasonable person would deem necessary to stop that threat.
With him on trial, I don't have to prove she intended to continue her attack. With him on trial, you have to prove his belief that she would attack is unreasonable, and/or that the kick was more force than was necessary to stop that attack. Unless he confesses that his intent here was to cause harm rather than to stop her from causing harm, I don't think that burden can be met.
I think nevertheless kicking someone without the clear intention of equivalent physical harm can not be justified.
That is incorrect. The standard is not "clear intention". The standard is "reasonable belief". As soon as she began striking him, she opened the door to a reasonable use of force to stop the threat she posed.