D.B.
I think you absolutely cannot bring this up to your co-worker. What goes on in the school must be dealt with by the school. If your daughters had a playmate and something occurred, you could CONSIDER discussing it, but even then, it's kind of micromanaging friendships. Besides, you cannot tell this mother what her daughter is doing - you only know what your daughter says the girl is doing. Kids are notoriously poor reporters, and even if your daughter is 100% accurate, what's the next step. The mom goes to her daughter who says it isn't true? Then you have a giant she said/she said situation. And the other girl feels tattled on and takes it out on your daughter? Even if you were friendly with the other mom, you can't do it - you are coworkers and this is not a work-related issue.
You teach your daughter to advocate for herself - that can mean speaking up to the other girl, or it can mean teaching her to smile, shake her head in a baffled way, and walk away (letting the other girl know that it doesn't bother your daughter and conveying that she's being annoying and immature with her repetition, without saying it outright). Or she can go to the teacher.
You can certainly go to the teacher, but if the other girl is doing this without being overheard (which is what most bullies or sarcastic kids do), you're going in awfully late with the info. So you teach your daughter to steer clear of the annoying child, buddy up with the nice kids (which is working), and develop confidence in her abilities. She doesn't have to go to every fight she's invited to.
If you get involved, it takes away your daughter's power and it will absolutely ruin your working relationship. If I were the school administration, I would absolutely not want my staff blurring the lines with parental issues.