He may have seen Gina before, but he wasn't that close to his kids, so he may not know her friends. Also, I read somewhere that he thought Gina was older. Now if he knew her a his 14 year old daughters friend, he would have known thar she was 14.
If he claims he didn't know how old Gina was, I don't believe it.
Anyhow, it doesn't matter. After he found out, he obviously didn't let her go. And I presume he did know her as his daughter's friend. According to what has been reported about Gina, that's how he lured her into this car. He promised he would take Gina to see his daughter.