The chances of getting the charge dropped to Section 20 are negligible. See paragraphs 50 to 67 here:
http://www.cps.gov.uk...s_against_the_person/
The 'starting point' sentence for a first-time offender, convicted (after a trial) under Section 18 is 5 years imprisonment. Other than under extremely exceptional circumstances, judges must look at a sentence of between 4 and 6 years imprisonment. However that sentence can then be reduced by one third if there was an early guilty plea. The time actually spent 'inside' is normally half of the nominal sentence.
So, at the bottom of the scale, the sentence will be 4 years, less one third, equalling 2 years 8 months. So that would mean your mate being 'out of circulation' for 1 year 4 months.
See the table on page 13 here:
http://www.sentencing...person_accessible.pdf
Chris