stay in contact with her teacher - make her your new best friend. seriously, she is your best ally in this situation. she's aware of it, she's on top of it. she's doing her part sounds like. if you have concerns or don't know what to do, ask her. ask if there's anything she needs you to do, any way you can help. keep in mind that if, at any point, you go above her head, you run the risk of no longer having that ally for your daughter. obviously in some situations that is warranted, but really it should be a last resort.
i would also be teaching my daughter things to say. such as, "that's NOT NICE and i DON'T LIKE IT!" and walking away.
you can't "teach" this child not to hit girls. he shouldn't be hitting anyone, #1. if his parents didn't teach him better, and the teacher is doing her best, it's not going to be you that magically makes it happen. work WITH your daughter's teacher and trust that she's looking out for her students - she obviously is.
also have to play devil's advocate just a bit here. i have a little boy in kindergarten. he is NOT mean spirited and is by no means a bully - but he is still adjusting. he was almost 6 when kindergarten started (is now), but was still emotionally immature. he has a very hard time controlling himself and is in the "safety seat" pretty often (i would say at this point we're probably down to once or twice a week - but at the beginning of school it was almost every day). this could just be an emotionally immature little boy who is still learning. yes, the comments are hurtful. but he is just a little boy. it's really on the parents more than the little boy. i can't imagine what my son would be like if we hadn't been drilling into him from day 1 how to act. just saying.