Lucius was testing the diary. It had, after all, been locked in a box for the last fifty years. He had to make sure it still worked as intended before giving it to the Weasley girl, didn't he?
So he'd brought in some minor Ministry lackey who nobody would miss and performed the necessary spell to transfer the man's soul into the book. It would be enough to bring Tom Riddle out for an hour, if all went to plan.
The fact that Lucius knew the sixteen-year-old Tom Riddle had been a strikingly handsome boy had nothing to do with his insistence on testing the diary, of course. But he did have plans for the hour they'd spend together.
Ginny was kept in seclusion after she was brought out of the Chamber. Her family told everyone she needed time to recover from the emotional trauma of her ordeal, and that she would be staying at home where they could look after her, until the next term started.
This was, of course, true. She had been through a lot, and she was very young. She would, undoubtedly, need time to recover from such a nasty experience.
But it wasn't the whole truth.
The Weasleys didn't want anyone to see her, because then they might work out that she was suffering the after-effects of an abortion draught.
Lord Voldemort had concluded that it would strike a most effective blow to the morale of Dumbledore's side if he could just recruit one of his most outspoken supporters. It would have to be a Weasley: surely in such a large family, one of them had the intelligence to choose the right path?
The Dark Lord's spies told him that the third son of the family, one Percy Weasley, appeared alienated from his brothers and sister, and spent more time at his desk in the Ministry than he did at the family home. He was the one to work on, then.
Lucius Malfoy's son had become most attractive.