ok, well, you see i am ok i really am, but...i cant live with my best friend dating my ex!?

my best friend rebecca is going out with my ex zack and she asked me if i was cool about it and i am its just so weird...and he looks at me like "ha!i am going out with your other best friend!see what you get for dumping me *****!" and hes making rebecca act so weird...like hes making her change for him! i keep telling her that she isnt going to be the same person when she goes out with him but she acepts my concern and does nothing! the other rebecca is spredding rumors like he asked her back out and rebecca keeps asking,"would he really say that?" and i keep telling her "he might be my ex but hes a nice kid...he wouldnt cheat on you." but i also keep douting myself... i think hes using her for something but i have know idea what! i dont want to tell rebecca because she will probly get mad and think im jealous wich im not because i dont like him any more and i know it! i am worried about rebecca and a little hurt that she would do it even if i said it was ok. i need help, i need help