Do you think it is right if I don't defend my friends from a crazy girl?
Would you defend your friends if you had to from this crazy girl?
No, I would leave them to deal with it. They are all my age now they should know what to do.
A crazy girl is something that everyone needs to defend themself from. But not defending your friends even when they should know how to already is not acceptable. Be there for them even at this point, so they know you are dependable.
If you want I can defend them but they shouldn't care. They don't need my help.
It doesn't matter if they don't need your help, you should still offer it because they are your friends. It is about the willingness to be there for your friends that matters.