Is my best friend sleeping with her own dad?

This might sound weird, but I'm sure there is something strange going on. She is 24 (same age as me) and I don't know how old he is, I'd say around 45. Everytime I see them together, they're sitting next to each other with his hand on her knee and her arm around him. They look into each others eyes but not the usual parent/sibling look. More like a lovers look. And when I went round on Christmas day to give her my gift, she kissed him on the lips in thanks for a gift he'd bought her but it seemed to linger for long period of time. When she walks by him he always slaps her and laughs, yet she doesn't say anything but laughs with him. I've told my mom my suspicions but she said I can't make allegations about people without any bit of fact. But doesn't that sound to you like something is going on between them?