He sounds kind of messed up. Maybe he thinks that people will find him more attractive if there's a girl pining over him. Rather than worry about why he's behaving this way, I think you should just be grateful that you're no longer with this loser.
As for telling your friend, I think it really depends on how you think she'll react. If she's somebody who would be considerate and thank you for the warning, then I'd say tell her. She might still ignore you, but at least you'll know you've done the considerate thing by giving her the heads up. On the other hand, if you think she would give you a hard time about it, then I wouldn't bother. You're not obligated to subject yourself to that for somebody else's sake.
Don't give up on love because of one bad relationship. There are plenty of good people out there. Just try to be more cautious with who you choose to date next time and don't get too serious until you know what type of a person you're dealing with.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy