in the event a honest guy tries to reference a legit tip and the attacker's tip, he'll detect the contradiction and won't do it. Therefore, the attacker's subtangle will be abandoned.
This requires not just an honest guy, but a diligent one as well.
The risk is that honest guys will be lazy and rely on others to go far back in history to check all tx for double spending.
The lazy guys risk that their tx's will be abandoned, because the majority of the nodes won't reference them.