I think it’s probably very natural for a 16 year old to be flattered and pleased that she’s caught the eye of an older man. And for things to get too far out of hand, because of his behavior and her immature emotional state.
With mom’s boyfriend who is living in and having sex with mom? No. My dad died when I was 8. My mom met my stepfather when I was around 14-15. He did not live with us or stay over, ever. But, if he had shown interest in me, my reaction would have been to be sick to my stomach and repulsed. That is what hers should have been, but for some messed up reason wasn’t.