Because they destroy that precious keepsake and know her friend being bullied for a long time now. They deserve it,
Breaking the rules? heh, screw that, after all you can't be a 'too good' person if you want to survive in this world.
And no, bullying is not a sign that you're strong, quite the opposite, you're just a weak and a coward who only can make a mess someone weaker than you and can't face someone stronger than you.