How to handle it when your man cheats with another woman.?

If an engaged man cheats on his fiancee with another woman who is single and then after their make out, he decides to ends the visits and encounters with her saying they just kissed once and he cannot do it again...anyhow every time he sees her at parties or hangouts, he always looks at her, flirts with her, talks to her... but my thing is this if the woman is talking to other men, this engaged man stares at her listening to what she is saying to others and her behavior. at times if she smiles at him, he does not smile at her, but he keeps looking in her eyes the whole time. why is this? and could it be he is looking cause he is guilty with cheating on his fiancee and stares at her cause he is scared that she may take control of him or be obsessed with him, or is he staring at her most of the time cause he still wants her and likes her? Or does he stare at her cause he knows she wants him and he knows looking at her distracts her, but that is just it, why would he look at her even standing beside her when she does not even see him, why woudl he listen and be attentive to everything she does?? he can be across the room from her or standing right behind her and still gaze at her eyes, her chest, and observe her and then talk, smile, flirt, and stare more??