Having heard both sides of this and of what happened from both of them, I would say that they may want things so different and communicate so differently that it won't work. But that is hard to say. I can tell you that I know Sam has always told him what she is and isn't willing to do, and to my knowledge she hasn't lied to him about maintaining anything monogamous or more serious than they have had up to this point. But Cris, I think, wants things to be exclusive. He wants reassurances that Sam doesn't care for Neil anymore. In short, he wants things that, from what she's told me, she refuses to answer and has been clear she can't offer guarantees on. If absolute reassurances of physical and emotional exclusivity is a requirement, I don't know when she'll be able to do that for him, but him constantly pressing the matter and then assuming things are the way he wants them to be when she's told him the contrary isn't helping matters.
I'll tell you what I told her. I think Cris wants a completely different version of my sister than the one that exists in reality, whose wants and whose heart match his precisely. If he was willing to wait for her to agree, on her own, to more closeness and a closed relationship, I think it might all work out. But I don't think he has the patience for that. He cares too intensely about her right now, and that culminates in ugly events like the one that just happened, where he pressed her on a point she won't concede and then she pulled even further away from him. He's an aggressive person, it's in his nature, but I don't think aggressiveness is a very good match for someone who prefers a more relaxed relationship. I think the fact that she never left my brother despite him also being a likely bad fit for her is a good example of that. Neil never pushes anyone on anything.
I don't think it would matter if you did tell him to run for his life. I don't think he's capable of that just now. But do I think this relationship is good for either of them? No, I don't think so. And I think that unless he can give her the time and breathing space she needs, it can't work.