my dumbass best friend and her cheating d*ck of a boyfriend

my friends bf dumped for some girl around christmas then after 3 days asked her out again she said yes then at new years he dumps her again and does the same thin this weeks she finds out that the 6 months he has been out her he has had one girl steady and countless other girls and he has been sleeping with them and we tell her this and then he denines it and SHE BELIEVES HIM and gets back with him

what do i do to get her to see sense ?