It is a 19 year old girl they are talking about, not a big man trained in martial arts.
If the chap was a stalker who'd bothered her in the past, it is not surprising that she felt angry and afraid and lost control. Even if she kicked him when she was angry, in the light of the situation that he attacked her first, planning to make another one of those ugly cases that keep appearing in the news at frequent intervals, she shouldn't be fined.
Once you're kicking someone in anger it's hard to stop unless you're trained to fight. So what was the girl's mistake? That she got angry at her attacker? That she decided to defend herself in anger?

Now I don't know British laws, but hypothetically, let's say that the drunken man had hit some rock when he fell down, broken his skull and died? What would've been the girl's mistake then? Not kicking him in a safe direction?