Even the Crown Prosecution Service's own internal document admits that 'attempted murder' is a difficult allegation to sustain. See paragraph 71 of this document:
http://www.cps.gov.uk/legal/l_to_o/offences_ag ainst_the_person/
However that same document also makes it clear that kicking the victim's head is regarded as evidence of 'specific intent' to cause GBH (see paragraph 65), which means that Section 18 will be the appropriate charge, and not Section 20.
For a Section 18 conviction, where "the victim suffered life-threatening injury or particularly grave injury (where the offence was not pre-meditated)" a judge is obliged (unless there are genuinely exceptional circumstances, which he or she must explain in open court) to sentence a first time offender (who is found guilty after a trial) to between 7 and 10 years custody. (The judge has to work from a 'starting point' of 8 years imprisonment, and then move the sentence up or down, depending upon the circumstances).
However, a reduction of up to one third of that sentence can be given for an early guilty plea. The relevant document can be consulted here:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Based upon that, your friend is likely to be sentenced to around
5 years imprisonment. If so, he'll be released on licence after a minimum time 'inside' of 2 year 6 months (or a maximum of 3 years 4 months). Up until the end of the 5 year period he'll have to attend regular probation appointments (and take part in any courses which the Probation Service deem to be appropriate). He'll also have to comply with any licence conditions. (Those always include things like not tra