If a weapon was used (but which only came to hand at the scene), or the offence was pre-meditated, or very serious injury was caused, or permanent disfigurement resulted, the starting point goes up to 5 years, with a range of 4 to 6 years.
If the offence was pre-meditated and involved the use of a weapon carried to the scene for the purpose, or the victim suffered life-threatening (or very grave) injury, the starting point is 8 years, with a range of 7 to 10 years.
If both of the circumstances from the previous paragraph apply (i.e. a weapon was carried to the scene and the injuries were very grave or life-threatening), the starting point is 13 years, with a range of 10 to 16 years.
So, taking the bottom of the range for the least serious type of this offence, and knocking off a third for an early guilty plea, it's likely that your friend will be sentenced to at least 2 years imprisonment but, if the offence falls into one of the more serious categories, the sentence could be much longer.
Source (Page 13):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Chris