(2-part post):
The Crown Prosecution Service will initially have to decide, based solely upon the extent of your friend's injuries, whether the charge should be 'assault occasioning actual bodily harm' or 'inflicting grievous bodily harm'. (An alternative wording to the latter charge is 'unlawful wounding'). Based upon this legal guidance (which is the actual document CPS staff must refer to), 'GBH' seems to be the more likely charge:
http://www.cps.gov.uk...s_against_the_person/
(Paragraphs 45 to 58 apply)
If the charge is 'GBH' (or 'wounding'), the CPS then have to decide whether to add 'with intent' onto the end of that charge. (Paragraphs 59 to 67 of that document apply). The use of a knife might be sufficient to cause that to happen.
Here's the document that judges must refer to when considering sentence:
http://www.sentencing...inst-the%20person.pdf
(See page 13 for 'GBH/Wounding with intent', page 15 for 'GBH' and page 17 for 'ABH').
It should be noted that the sentences there refer to first-time offenders who are convicted after a trial. An early guilty plea can see the sentence reduced by one third. Previous convictions can push the sentencing range higher.