I don't know what's right or wrong?

Uhm. He sounds like a jerk. If he treats his present girlfriend poorly then what would make him act differently if you dated him? Nothing. Don't get involved.

Don't get involved with him. He sounds like he clearly isn't worth your time. you shouldnt have to MAKE a guy like you, because he should accept you for who you are rather then who he thinks you should become. out there is a guy that will love you for you, and think that the freaking sun rises and sets over you.

but aside from that, also be careful about getting involved with a friend's ex. even if she said it was OK, she may still have some feelings for him in the long run. so basically, stay out of the situation, move onto a better guy, and make sure you keep the friendship going with your friend.

hope that helps :)