My thought is if he was at the top of the slide and was shoved and thought he might fall it could be a kneejerk reaction to turn around and smack someone before he had though it through at all. If they were in a queue at the bottom that's a different matter, but your daughter did instigate it by shoving him first and I'd be more worried about addressing that, because you can bet your life that she won't shove him again now there has been a repercussion for her behaviour from him, but you seem to be seeking redress from that when it might not be necessary.
If this kid is a bully and picking on your daughter, fair enough ask for a meeting but if she often bullies him, then take it she has just learned a valuable life lesson.
There's not really enough detail in your post to know which way to view what happened.