Both kids should have been called in to the principal's office- but the big question is did the teacher SEE the other kid push your friend's son? Have your friend find that out first.
If the teacher saw what happened, clearly the other kid started it. If not, it's your friend's son's word against the other kid, which is trickier. If I was your friend, I would make it VERY clear to both the principal and the teacher that they will not tolerate their son being bullied at school- make sure they use the word 'bullied'.
If they have actual proof of other incidents where this other kid has bullied or gotten physical, I would contact those parents and ask if they can mention it to the principal, just to back up their son's story.
They should talk to their son- was this just a random thing that only happened because both kids wanted the cone at the same time- or has this other boy been seeking out your friend's son and bullying him?
There is a HUGE difference- and honestly, the other boy doesn't sound like he was a whimpering puddle of terror- he stuck up for himself and gave as good as he got. I am not saying it is GOOD to kick someone- but at least it sounds like he wasn't too intimidated to do anything!
I know how upsetting things like this are- BUT- this is kindergarten. Boys get overexcited and out of line - even good kids. Sometimes bullies do grow out of things- I say this because a kid my son had an incident with when he was that age, has really changed and is very well-behaved now.
You don't want to go crazy over this- but they should make it clear to the principal and teacher that they expect the situation to be monitored and that if there is one more incident, they will go to the District and the school board.
Last but not least- call up the other kid's mom! She may not be surprised to hear what happened- or she might be appalled! She may just think another boy kicked her son for no good reason! Without communication, you can't make good decisions. Maybe if the two boys had some one on one time at each other's houses, they would become friends and the situation will end on its own. Don't discount that kind of solution without trying it!
Good luck to your friend!