As a man who has been in this situation i strongly sugguest you talkto him before it's to late. My now current ex waited almost 6 minths before saying anything to me and by the point I had already attempted to move on. It just further complicated my own relationships as well as my relationship with her. Overall it ended up not being well for either of our mental health. So please if you really care about this man reach out and speak to him. Communication is key in any sort of relationship whether it be friendship or something more. Explained what happened how you felt and maybe if u can identify early signs of it to help him recognize the situation. I 100% would have never moved on if i would have known then what i know now.