No, you're not. You were the responsible one in this situation. I don't know the girl but based on what happened this time, she sounds irresponsible. My daughter is only 10 months old, but we tell her off if she plays up and if she is being naughty or messing with things she shouldn't, she has to sit on mine or my partner's lap. She doesn't like it, but that doesn't matter. So what if her son doesn't like being held in one place, she should be the boss in the situation, not the 1 year old.
This type of parenting is what causes the terrible society we have nowadays where parents are scared to tell their kids off in case the kids hit them and teachers can get sacked for shouting at a pupil (no joke, a teacher at the school i went to was suspended for shouting at a pupil).
Ok, maybe you were wrong for disciplining HER child, but somebody had to.