I've been reading (and occasionally writing) court reports for around half a century and, based upon the limited information in your post, I would think that a prison sentence is unlikely.
However there are lots of factors which a court would have to take into account which are unknown to us. For example a single blow would be regarded far less seriously than a sustained assault and a level of injury only just that of 'common assault' (such as a minor abrasion) would be regarded as far less serious than one which fell only just below that required for a 'GBH' charge. Similarly, if your partner went to the restaurant with the specific intention of attacking his victim, rather than simply acting spontaneously, that would be regarded as making the offence more serious.
See pages 177 to 180 here to read exactly what the magistrates will have in front of them when determining the appropriate sentence:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf
Remember though that custodial sentences of up to two years can be suspended, so even if the court does pass a custodial sentence your partner wouldn't automatically go straight to prison.
From what little information we have though, I'd think that the very worst outcome might be 4 months immediate custody (which actually only leads to 2 months 'banged up' anyway) but I think that a sentence avoiding immediate custody is far more likely.