NTA as long as you didn't attack the mom. She's speaking this way of a stranger and her daughter 's friend's friend.
Am I the Asshole?
I think !amitheasshole is deleted by creator.
You're never the asshole about your feelings. From what you posted it seems you didn't say anything to her. You probably made the right call, as I doubt you'd get anything but defensive posturing in response.
I'll say it though; Liv's Mom is a judgemental bitch.
NAH, though I deem her wrong in her attitude and am at least glad, that she didn't say that into your daughters boyfriend face. You don't know her background. What, for example, if she once was [nearly] raped by somebody she deemed a friend? Or …
Though it is good to trust your daughter and her friend. And stand up for them and thus don't let the other moms fear spread. My advise would be to not judge her, alone on the words spoken, yet don't let her fear and prejudice spread in the direction of your daughter or her friend as well.
And: never be too trusty though (yet the situation you described sounds public enough to indeed be safe).