Assuming that the assault was not pre-meditated, and that it resulted in "in minor, non-permanent injury", the 'starting point' sentence for ABH is a 'high level' community order, with the judge being expected (other than in really exceptional circumstances) to pass a sentence between a 'medium level' community order and 6 months imprisonment.
However those sentences apply to a first-time offender, convicted after a trial. Judges are told to regard head-butting as an aggravating factor and previous convictions can push the sentencing range higher. Against that an early guilty plea can see a sentence cut by one third.
If the judge decides not to go outside the sentencing range I've referred to above, it's likely that he'll consider a 6 month sentence, reduced to 4 months because of your partner's guilty plea. So he'd actually spend 2 months 'banged up'.
However if the victim's injuries were more serious (e.g. a fractured cheekbone, requiring surgery), or if the judge decides that more weight needs to be given to your partner's previous offences, a sentence of 8 to 12 months (with half of that time being spent 'inside') might be more likely.
The chances of a non-cutodial sentence seem close to zero. Prison seems to be a near certainty, and it's unlikely that your partner would be considered eligible for 'tagging' a second time.
Chris