Ok so if i understand correctly, someone pushed your partners friend, so your partner threatened this male (or was it more). All were arrested and later charged with Sec 4 Public Order. (to be 'charged' with a criminal offence there has to be sufficient admissible evidence for a prosecution to likely to succeed in a conviction).
His solicitor should be advising him accordingly and whether to enter a guilty plea at his first court appearance (there is discount in sentencing for early guilty plea's). This offence is whats known as a 'summary' only offence and will be heard at the Magistrates court (as opposed to the Crown Court) where the maximum sentence is six months in prison and / or a fine. It is more common in this day and age for first time offenders to be sentenced to a Community Punishment Order (formely known as Community Service) along with a fine which is dependant on the defendants earnings. However, if found guilty the courts will assess all the aggravating features, such as being drunk, any weapons, the vulnerability of any victims etc. The court also considers any mitigating circumstances i.e. the defendant thought he was acting in self-defence.
Your partner should direct all queries at the solicitor, who would have heard all the facts of the case etc and will be aware of current sentencing trends in your court area's.