I would normally agree with you (and have been friends with both sides of partners that had an ugly split before) were it not for this: "Her ex said she cheated for a while until he found out, and she said he harassed her to an extreme that she almost got the cops involved."

Do you believe her? If you think she is lying then I would question the friendship anyway. If you don't, I think it is time to consider the message your friendship to him is sending to her and others who know her side of things. It sounds like you are choosing to hang out with your best friend's abuser. Assuming you are in the states, if it reached the level where she almost had to call the cops, that means she felt her life was potentially in danger or her safety was compromised. While cheating is a form of emotional abuse that can put someone's physical health and psychological well-being at risk (and she absolutely shouldn't have cheated), harassing someone to a level that requires law enforcement involvement is a different level of abuse and a real safety concern for her (by definition aggressive and abusive). It sounds like you've diminished that fact because siding with one of them means recognizing at least one of your friend's flaws (either his or hers) for what they are and losing a friend who you enjoy (either her or him). But by passively ignoring the problem, you're probably going to lose out too.

How safe would you feel if you knew your best friend (who likely knows details about your life) was still on really good terms with someone who stalked you (and yes that is what harassment implies)? And that is assuming they are both telling the truth. How much worse is it if he also lied to you or others about the cheating to justify harassment? Either you believe that she is lying (in which case why are you still friends with her) or you believe that you are friends with someone who harassed your friend. You can't ignore that and still be a good friend to her and to assume she wouldn't be talking to you about how it feels to see her "best friend" diminishing that experience isn't giving her enough credit. Would you rather she didn't say anything? Best friends communicate with each other about these things. By not changing anything you are saying you heard someone harassed your best friend and that didn't change how you saw the person. So yes, if you want to stay his friend then she is right to reconsider how good of a friend you are to her. Even if you promised not to ever talk about her to him, you're asking her to trust him and you and both of you have made it clear her needs and safety aren't a priority. You can't call her your best friend when you are regularly hanging out with someone who harassed her. Tell her you agree that she has some reasons to be upset, but that you also care about his friendship and want to maintain both. Say you are open to talking more about why this is hurting her and hearing her side. See what she says and then make a decision. But also accept that if you don't cut him off, she is absolutely justified in cutting you off, especially if there are any lingering safety concerns here. If she really is your best friend you wouldn't be blowing any of her feelings off, even if they were unreasonable, you'd be addressing them out of care and consideration for her. So that is the part that sounds insensitive. It sounds like you just ignored the entire break-up and doing so means ignoring the fact that your best friend was harassed. I'm not saying she is completely innocent here or that he is a monster. I'm just saying if you know her and trust her and want her in your life it makes sense to take the time to understand why this is still weighing on her. If you want to keep her as a friend, you need to be at least open to hearing her feelings and validating concerns, even if you do want to keep him in your life.