I have been on both ends of this.
Dd can sometimes be a bit of a "bully" to younger children, but she gets her comeupance from older children we know!!
All i can say in defence of the other Mother is it is very difficult and embarrassing to deal with, esp. in large group of people. Sometimes i feel like my dd is a bully and all the others are little angels and that everyone is tutting behind my back at my parenting skills ( i know this isn't true, but at the time you can feel like that).
I always make dd apologise and give her a bit of an explanation as to why she shouldn't do it, and then keep by her side to ensure it doesn't happen again.
When she is on the receiving end i feel i have more sympathy for the parents, and i say don't worry, shes been the same to others, so that they don't feel embarrassed.
If parents don't actually witness the incident i have in the past, told the child that its not a very nice thing to do and then made a point of something we can all play with together nicely.
It is upsetting when your little one gets hurt, but i would put it down to experience, i'm sure your son will have a patch where he does something similar, or snatching toys etc
If it continues happening of course you would be upset, but for now i'd hope your next meeting is less eventful!