When sentencing someone for common assault, magistrates have to decide whether the case falls into Category 1, 2 or 3 (with Category 1 being the most serious). While a custodial sentence for common assault is theoretically possible, it's only available for offences at the very top end of Category 1 (e.g. where an attacker had deliberately sought ought a vulnerable victim, such as a child, and then launched a repeated attack upon him). There's absolutely no way that a custodial sentence could be passed under the circumstances that you've mentioned.
Based upon the limited information you've provided, the matter will probably be seen as a Category 3 offence, with a fine (or even a conditional discharge) being the most likely outcome.
For further information, you can read exactly what the magistrates have to stick to, here:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf
(See pages 189 to 192)