Yes Sorry, I intended to say that the full circumstances are still being investigated, rather than the injuries. That is what will decide if S18 or S20 is the charge. There is also the possibility that the police will charge both suspects under joint enterprise rules, that needs both to face the same charge so both will be charged under S18.
Your friends really need to see a lawyer ASAP , S18 is always a prison sentence , S20 can be a prison sentence depending on how serious the case is.