They decided to separate. Harry would search for the diadem while Hermione and Ron went to the Chamber of Secrets because, in a moment of pure genius, Ron had remembered that Harry had destroyed one of Voldemort’s Horcruxes with basilisk venom by using a tooth. And since nobody else had since been informed about the entrance to said chamber, never mind been able to speak Parseltongue to unlock it, the giant beast had been left to rot and there were undoubtedly skeletal remains for them to use.