How should I approach her about this?

So my best friend and her boyfriend have arguments sometimes like most couples do. But lately she's been finding suspicious messages in his phone. This hasn't happened once but multiple times. I know this because she tells me that she's done with him, sends me screenshots, etc. etc.. It's either the girl who confronts her with the screenshots is lying or she just lets it go and gets back together with him. I'm sick of this, but I don't know how to approach her about the situation. "The situation" as in her telling me this and telling me that she's done and bringing me into their argument, I could honestly careless anymore, call me a bad friend or whatever. Either way I send her advice and she doesn't take it and I'm not trying to waste my time on this anymore. How should I go about telling her? She recently did it today, and I'm feeling like I'm sugar coating stuff and letting it slide and I just feel like I'm being a bad friend by doing it but I don't know what to say. Any advice from the blunt, don't-take-shit-from-anyone type woman of eve?