Worried

This is literally none of my business but I’m just so grossed out. My friend is 16 (I’m 17) and she’s dating a guy that is 20. Like what business does a whole ass adult have with a girl in high school?? She likes him a lot but I have a weird gut feeling like he’s taking advantage of her naïvety. She didn’t listen to me when she asked for my advice before they got together and i just worry about her so much. Ik it’s only 4 years but she isn’t even fully developed yet (not that she looks like a child but her brain and shit) just needed to rant