You've not provided sufficient information for a meaningful answer. The judge has to consider both 'culpability' and 'harm'.
The use of a weapon (including kicking someone's head) or a planned attack, for example, would probably mean that 'culpability' would be regarded as 'high', whereas a spontaneous pub brawl (where no weapon was used) would mean that 'culpability' would be 'low'.
GBH only involves 'very serious' injuries but there is still a range of severity to be considered. A broken jaw, for example, might see 'harm' rated as 'low' but a sustained attack, or one causing life-changing injuries, might see it rated as 'high'.
If both factors are considered to be low, a non-custodial sentence can be passed but a prison sentence of up to 51 weeks is possible.
If one factor is low, but the other high, a prison sentence of 1 to 3 years is the available range for the judge to apply.
If both factors are high then a prison sentence of between 2½ and 4 years must be passed.
(The sentences above can actually be reduced by up to one third because of the 'guilty' plea)
So, if your friend was (for example) only involved in a pub brawl where he broke someone's jaw, but didn't 'glass' them or kick their head, he
might get a non-custodial sentence. However his previous convictions won't help (particularly if they're fairly recent), so he might still get a short prison sentence. If it's anything more serious he'll definitely go to prison.
A character reference could possibly make bit of a difference but one from his employer (saying how good a worker he is and that his job remains open to him if he's not sent to prison) would probably carry more weight. This might help. (Change 'Magistrate' to 'Judge'):
http://www.ehow.co.uk/how_8162314_write-character-reference-court.html
See pages 7 to 10 here to see exactly what the judge must refer to when sentencing:
http://sentencingcouncil.judiciary.gov.uk/docs/Assault_definitive_guideline_-_Crown_Court.pdf