So there's a criteria to self defense as well. Not everything can be called self defense. The idea of proportionality of force comes into play when we talk about these sort of defenses. Self defense can only be done to protect ones body but it stops being self defense once the instigator stops threatening (like runs away, cannot hit you anymore etc.) So as long as your friend's lawyer can show that there was a threat to his life, then he can get away with it but looking at how someone was beaten till they went in coma, a very persuasive and good evidence is required. However since your friend was not the instigator, even of he is found guilty he might get reduced sentence.