What should I say to my friend?

So my best friend has been dating this guy off and on for about a year and a half. He's cheated on her 4 times during the course of their relationship and every time he leaves her in worse shape but he comes back to her begging for forgiveness and she takes him back. The first time she took him back I was okay about it cause sometimes people make mistakes and need to realize what they had after they lose it but the second time I told her not to give him a second chance. She did anyway and even though I didn't approve I was supportive cause she's my best friend. The third time he did it she swore she wouldn't take him back and their break up actually lasted for a few months so I thought we were past this but last night she took him back. I don't know what to do at this point cause I'm sick of watching her get hurt. What should I say or do?