One of Those Mothers

I think I have turned into "one of those mothers". There is a little girl at Maddie's school that is driving me insane. Maddie is a little dramatic so I am not sure that everything she tells me is exactly as it went down, but yesterday I had my fill. Everyday she tells me of the "problems" they had during the day. It is usually something small, but today she wrote on Maddie's shirt. Not a little bit, but several huge streaks. Maddie waited until we got into the car to tell me about this. I decided I was taking this little girl on myself. So today when I picked her up at school I asked the little girl is she wrote on Maddie's clothes. She said, "I don't know." Okay I know these children are only 5, but they know if they wrote on someones clothes. I asked her not to do it again and left. Now I am concerned that Maddie is going to be one of those children who doesn't stand up for herself. I have told her over and over not to play with this child, but she feels sorry for her because no one else will play with her because she is so mean. Maddie lets this child manipulate her into playing with her. Maybe I am taking this too serious but I believe the adult we become has a lot to do with our personality development as a child. By no means do I want Maddie to be the bully, but how do I instill in her not to take this crap. I don't want her to be a follower. I want her to be comfortable with who she is, but she isn't comfortable with this if she tells me about is daily. Any suggestions? I won't always be there to be the MOTHER that butts in.

By the way---funny, but not funny....today when I picked brother up we got into the car and he said you big butt came to get me today. First we don't say butt, so I am not sure where this came from. So I told him we don't say butt. So he fixed it and said you big hiney came to get me a school. Should I take the hint?
Funny that he said this, but I tried not to laugh in front of him. Anyway.