If it was a one off, I would probably discuss with M how he could deal with it in school i.e. Tell the teacher or not spend time with that child so much if they didn't like their behaviour. I want him to be able to handle things himself wherever possible.
He is pretty good at this. In particular, a boy in his class stole his rubber, spent the day waving it at him and refused to give it back. He did try telling the teacher but she misheard and he didn't go back for another try. At the end of school and before going to his dad (who was picking him up), he went straight to the other boys mum and told her what happened. She promptly frisked her son, told him off and returned the rubber. Result!
If the pushing or whatever happened again or I was concerned and I knew the parent I would speak to them, but if I didn't know them I would speak to the teacher. I think what you did was fine. It was also ok for her to mention to the school as she may now be concerned about her childs behaviour.
Our school has asked in the newsletter for parents to discuss issues with them rather than confront the other parent and this is probably in response to some trouble between parents.