My best friend and this guy?

ok so my best friend and i are like sisters and we always will be. lately shes been hanging around and has become friends with this guy and i dont like him at all. there is just something about him that i cant pinpoint and i dont trust him. she knows i dont like him (which was so hard for me to explain to her) but shes going out to dinner with him tonight anyway. she keeps saying that they are only friends but its tearing me up inside. what should ido?