Having had a look at that it's worth noting that there is a concept called "joint enterprise"
If for example someone is murdered in a robbery - someone not involved in the actual murder but involved in the robbery - for example the lookout can be charged with murder under this.
I'm not suggesting your friend was a lookout but just because he didn't stab the person himself doesn't put him in the clear
Here's the guidance
http://www.cps.gov.uk/news/press_releases/joint_enterprise_guidance_published/
Again - a complex area and not one people on here can give a good judgement on based on very limited facts