I complement my own son a lot on the rare occasions I go to places like this. Stuff like "good waiting" and "excellent taking turns", partly for his benefit and partly because other mothers do start to model me and expect the same behaviour from their children, which helps me have a better time.
I don't like going with friends though - then I feel torn between talking to them and helping my ds have the best and safest time.
I think you did really well to challenge his behaviour both to him and to the mother. I'm sorry she didn't respond more positively, but I'm sure she thought about his behaviour privately when she didn't have her mates around.