Pushing somebody, when circumstances don't compel you to do so (e.g. because you could flee from an assault), is 'Common Assault', not 'ABH'. To see what the difference is, read the relevant sections of this document:
http://www.cps.gov.uk...s_against_the_person/
It's also worth noting that your friend could be correctly convicted of 'ABH', even if he didn't strike the blow which caused the relevant injury, if his friend struck such a blow. See the second paragraph of my second post here for an explanation of the legal concept of 'joint enterprise':
http://www.theanswerb...l/Question737849.html
(If the court believes that your friend and his mate attacked the shop worker in a 'joint enterprise', they can BOTH be found guilty of any offence relating to that attack, even if one of them didn't actually cause any injuries).
Your friend needs to get a good solicitor to review the evidence (including viewing the CCTV video) and to advise him as to the best way forward.
Chris