How do I stop liking him?

Well, if she broke it off with him then she doesn't want him. I dont want to say take her "sloppy seconds" but I've been in that situation before. If you care for him, I'd tell your friend (girl) how you feel first. If she's a real friend she'll understand where your coming from. If she gets mad, she's just jealous and she basically had her chance with him and it didn't work. I'd still stay close to the boy because obviously he makes you happy and you both have fun together. My best friends went through this and they're not as close as they used to be now but you have to do what you feel is right. I would keep hanging out with the boy and if the girl starts to act different or bitchy I wouldn't be her friend. Real friends understand everything and will be there 100% no matter what so maybe you should reconsider how close you and her are if she overreacts