Anny wrote: Johann says nothing to defend his girlfriend.
This scene shows conflict between the different sides of the poor boy. Johann Is a character that needs a leader, and follows everything that Marcus says, he looks up to him. So when the two people he cares about take different sides of an argument, he's torn between defending his girlfriend or defending his best friend, which is why he doesn't say anything; he doesn't want to make things worse. But he's in a position where somebody WILL get hurt. and staying quiet and not taking sides was probably, given the situation, the only thing a person like himself could think to do.