Bestfriend finally dating.

Okay so my bestfriend has never went out with ANYONE before. She's 16, well about to be this month. Well the guy she's dating I know him personally, he's kind of like my guy bestfriend, I know that he likes to mess around & stuff like that because he's shown me before. Well anyways he said he would change for her & stuff but I'm not sure. I feel like he would end up hurting her, and I've known this girl for about 6 years, I wouldn't want anything hurting her. But at the same time I wouldn't want to be right about him & just watch it happen, but I'm not sure, that's the problem. Also I don't wanna tell her what I think cause that would prolly hurt her and like I said this is her first boyfriend... What should I do?