Please, let this one go. It's not pleasant when children are unkind, but we cannot and should not be policing what other kids say to our children.
My guess is that if the girl was saying this in class, the teacher would correct her by pointing out that if she doesn't have anything nice to say, she should keep quiet. That's usually what I saw in my son's K class last year. On the playground, if a child complained that another kid said something to that extent, the teacher would usually advise the child to go play with someone else and that there are other fun friends to be had.
Teach your son resilience. My boy has heard me tell him on several occasions that "you will not like everyone you meet, and honestly, honey, not everyone will like you. That's okay. Choose to be with kids who you have fun with." In short, help them NOT to dwell on the insult, just say "wow, pretty bad manners, huh?" and move on. Your kids need that from you. Yes, it stinks to hear a story like that, but let's leave that molehill a molehill.
And if there is a consistent issue with this (her singling him out and picking on him repeatedly), of course, do bring it up in a calm conversation, but one offense? Let it go. They're five and six and not known for their manners-- but they are known to be speaking their minds and then changing them a day later. (the girl, not your son)
From your just-added SWH: work with your SON on helping him to speak up. Seriously, it will really help him. Talk to the school counselor or teacher and ask for help in encouraging your son to speak out. Role play using puppets and switch roles time and again (have him be the pushy/rude one as well as his own role). When my son was repeatedly getting hit, I found a moment to talk to the teacher about my son's reluctance to speak out. It was great; I didn't have to name names and she reminded the *entire class* about being safe with their bodies. This also cued her to watch out for the kid and she told me how she had handled the situation. It was much more instructive for my son when we went about it that way.