How to stop my friend becoming a sl*t?

my best friend was exactly like that. she was making out and having with anybody just to feel a little loved. she even had her first drunk on a bench in a park! i told her to stop being like that and tried helping her but nothing helped. her behavior changed when she met a really nice and loving guy who did everything for her. they have been dating for a year and i thought that slutty time was over. bt then she met the next real and broke up with the nicest guy on earth for him because she thinks she loves him. the guy she lef is heartbroken and she will be too because the guy shes with now is such a man. but noone can help her with that. she wont believe that all that wont work. you can only hope that your friend will meet a great guy like my friends ex or that shell realize that her behavior isnt okay. all you can do is wait. thats what i have to do.