Honestly, STOP! One reason for the repeated attacks could very well be that even though she seems "friendly" to you, she might have bad dog manners and easily provoke dominant/high strung dogs. Boxers do tend to be kind of bouncy and annoying to some dogs... even though, bad manners DEFINITELY doesn't excuse another dog from ripping up her face. Those dogs obviously aren't well socialized at all, and don't belong at the dog park.

*sigh* The thing I dislike about public dog parks is that any idiot can bring his aggressive dog in there, and rarely do they get caught. I can totally feel for your brother... Gonzo also got his face punctured and the roof of his mouth torn up by an aggressive Rottie at the dog park. My best advice to your brother is to find friends with well-socialized, friendly dogs to play with his pup! I never go to dog parks anymore, unless it's only to meet people that I know with dogs that I can trust.