Your own post, Ako72, uses the word 'retaliated', which clearly indicates that more than 'self defence' was involved.
Further, unless there was an obvious reason why your friend couldn't simply run away after having knocked the other guy to the ground, it is extremely unlikely that then kicking him could be classed as 'self defence'.
However the law does accept that in moments of stress (as when being attacked by someone) a person hasn't always got the time to carefully consider exactly how far his defensive actions may go. Lord Morris (in Palmer v R, 1971) said: ""If there has been an attack so that self defence is reasonably necessary, it will be recognised that a person defending himself cannot weigh to a nicety the exact measure of his defensive action. If the jury thought that that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought necessary, that would be the most potent evidence that only reasonable defensive action had been taken ..."
I would rate your friend's chances of acquittal as very low but it's just possible that a really good barrister could (by reference to Lord Morris's words and to other relevant case law) persuade a court of his innocence.
http://www.cps.gov.uk/legal/s_to_u/self_defence/