Hugh Hefner's ex-girlfriend Holly Madison thinks he is making a mistake by marrying Crystal Harris.

The 84-year-old Playboy founder proposed to the 24-year-old Playmate on Christmas Eve but glamour model and reality TV star Madison - who split from Hugh in 2008 because he did not want to get married - thinks he will regret his "hasty decision."

"I'd like to see Hef settle down. I just want it to be with the right person, and I feel like he's making a hasty decision. I'm not sure Crystal is the best thing for him. Crystal hasn't been around very long -- she's a mysterious character. No one knows much about her. Plus, she's very young, probably too young to settle down."

Holly, 31, also revealed she is worried aspiring singer Crystal is using Hugh to start her career and fears he could end up losing everything in an expensive divorce: "I've seen a lot of girls try to date Hef, and some have ulterior motives. I don't think it would be a bad idea for him to get to know her a little better. I think it's possible Crystal could break Hef's heart. They could end up divorcing, and she could take half his money. I'm not jealous that he'd marry someone else. I just don't want it to be a girl who's using him to start her career. Basically, I think he could do better than Crystal."

Crystal has denied Holly's claims. "I've never been happier in my life. I'm so excited to be marrying Hef," she said in a statement.