Should I step in and do something or watch my friend make another mistake?

My best friend Amanda broke up with her boyfriend Vince a year ago, they were together for 2–3 years. Before they met, she had a drinking problem & was having problems in her sophmore year in college. They fell in love and were kind of addicted to each other (couldn’t sleep without each other, constantly together, etc). Vince is an idiot, he sold drugs & painted houses on the side for a living. He and his idiot friends have also been to jail for drugs, DUI’s, and any other dumb shit they’ve tried to pull off. Basically not a good guy. Their relationship was verbally abusive but never physically abusive and she broke up with him once she realized that he was a bad influence. She dropped out of school, ran up a shitload of debt, and now is trying to pay it all off and get her life together. She has a new boyfriend she’s been with for about 5 months who is very stable. He’s a very nice guy with a good job and a bright future and now they are “in love”. Her old boyfriend won’t leave her alone and as much as she refuses to admit to me, they both still love each other very much. Their relationship is very sexual, although I’m sure there is alot of love there too. I know that she has already cheated once on her current boyfriend with Vince and may do it again, given the opportunity. I feel so bad for her new boyfriend, he is such a nice young man, very calm, never has any drama going on and he really cares for her but she cheated on him and I’m not sure whether or not I should tell him. And if I do tell him, he’ll probably dump her and she’ll probably go back to Vince. She is my best friend and I love her like a sister but she can be so stupid. Can I/should I keep her from making more stupid mistakes?