Help me deal with this stupid guy on facebook?

[xoxo]

So this is a guy friend I haven'ts een in a while but added me on facebook sometime ago. When he was single he kept trying to get me to fly and see him but I was not really...romantically interested so I said no but stayed facebook friends with him. Then he started dating another girl for a couple years now so I was really happy for him. Then one night he randomly messages me how they don't have sex anymore and all this other stuff....and asking me to come where he lives again? I would normally IMMEDIATELY block this idiot but I am friends with his girlfriend, and I don't want her to wonder anything or start drama. I don't know the girlfriend super well she's just on my facebook so I don't want to tell her about how her boyfriend is being. Should I block him or just leave it alone? Literally this guy posts to my wall 3 times a day and stalks me on instagram. I NEVER reply but he keeps doing it.