I think you need more information to even know where to start. What does he mean by ‘lots of complaints’, I mean is he exaggerating because you aren’t agreeing with his viewpoint about the first incident and he’s frustrated? I would ask for clarification as well as specific examples, and also talk to your daughter’s teacher to get their perspective. If she’s in a sport or other activity, talk to the coach or leaders there too. It’s totally possible that what they have labeled as ‘mean’ or rude is actually symptoms of her ADD/ADHD and they don’t get it, or don’t want to get it. Knowing the situations he’s referring to will help you coach her on how she’s coming across, get her perspective on the situation, and help her find other ways to deal with the situations. And how is either of you supposed to do better and correct this so-called meanness if this is the first you’ve heard of it?!
In this case, discipline seems rather silly. Sitting her and the friend down, having the friend explain how it felt to be called that name, and then coaching your daughter on apologizing and making amends would serve her better. IF the friend is even upset, maybe it was just the adult who overheard it who didn’t like it and then over-reacted?