Well I was in the same position last week - but it was my boy who did the pushing
This was the first time ever that I've seen him push someone, and same thing, I dont know if he was provoked or not, there was a toy that they were both playing with and I assume neither kid wanted to share- but that doesn't matter... Pushing is NOT ok!
I was mortified, virtually in tears that my ds (turned 3 a month ago) did this.
We went over to the little girl he pushed and apologized profusely. I swear I almost throttled ds. I was hurt, sad and totally embarrassed that my normally placid kid did that to someone. We left straight awY - no more playing for that day and he had to sit in the pram rather than walk.
So anyways I think the least any parent could do is apologize to the other child and mum. Whether the child understands that they did the wrong thing is irrelevant.