Andrea was furious at his father for telling her that, he wanted to tell her himself when it was time. He knew she wouldn't like the way things turned out, he was also furious when he found out Lucifer had taken over. He found out a week ago when they got an invitation to the coronation of the new gang leader.
Lucifer has no idea what he has put himself through, he was too naive to realize what his father just did, and now there is a war they have to embark on, Lucifer received a letter from Andrea informing him of the war, Andrea wanted him to be prepared he knows he would never be able to forgive himself if he attacks without letting him know.
"She deserved to know, God knows how long you wanted to keep her in the dark." His father says, he didn't regret anything rather he felt what he did was right.
"I was going to tell her." Andrea lied, he never intended to tell her.