There is no accounting for why some people have no manners. My first thought, an adult that acts poorly as she did- must WANT to be the center of attention. So, if she ever does it again, let her BE the center of attention.
I would stop what I'm doing and ask her if she has something to say to you outloud? Let her repeat her comment. Then, proceed to tell all the children why being rude and making unkind comments is a form of bullying. It's something they should not do.
Then, simply proceed.
Hold your head up and be who you are. Don't allow someone that kind of power over you. : )