Here's what was probably going on in the assailant's mind:
'Yeah, ee was looking at mah missus, I seen 'im. Wew, I'll do 'im, ee ain gunna look at mah missus an fink nuffink gunna appen. I'll fukken bo'l 'im.'
or
'...'ats the bloke give my mate a load of bollox the other night, innit? I'm sure ats the one. Wew, ee's gunna fink diffrent nah, innie?'
Any number of misguided, alcohol-impaired reasons could have been conjured up by the man who hit your friend.