Most Helpful Guy
Wow this is tricky.
Are you sure he's giving you all the facts? How do you know she's crazy and possessive, how do you know she's this bad? Do you know her personally, and have you seen them together... or is this what he's telling you?
If it's the last of what I mentioned, then tred carefully. If he's telling you these bad things and flirting with you, he may just be trying to get with you while he's with his girlfriend. He may be over exaggerating, and you may be convinced because of your feelings for him. It may not be as bad as you think it is. He may not seem like the kind of guy to lie, but he may just be a good liar. Don't let your attraction towards him cloud your judgement.
If, however, you've seen first hand that she's a terrible girlfriend on multiple occasions, with your own eyes and ears, then you need to talk to him seriously. Don't flirt, don't do any of that stuff, just talk to him about his relationship with her, and ask why he's still with her. If you feel it's destructive, say so. Don't make him leave for you, to be with you. Make sure he's leaving for himself, to get out of a bad relationship. You don't want to be the girl that steals boyfriends. After they break-up give him some space, and then when the dust has settled see if he's interested in you. Don't let him jump from one bed to another, just like that.
Whatever you do, don't be the girl who gets with a guy who's not single. If he cheats with you, it can only end badly.
Most Helpful Girl
That's wrong and very selfish, leave him be, if you can't be friends without wanting something more then definitely end the friendship, it's his choice who he wants to be with and wether he wants to be with his current gf or not so don't say anything about and let him be with his gd, it's just unfortunate timing you both fell for each other but it happens even when your in a relationship, there is more than 1 person out there for us.