I hate my best friends girlfriend

so I’ve been best friends with this kid since 6th grade and were now in 10th. so in the middle of 9th grade he really started liking this girl, so being a good friend, I convinced her to hang out with us (him) on night and now they have been going out ever since. I’ve never had a problem with her until now though. She cheated on him TWICE, 7 months into the relationship…but told him 13 months into the relationship. she also broke up with him 3 times, twice sober and once drunk, but still. He just keeps on taking her back and he doesnt realize that she is just with him because he gives her all of his attention. So, she doesnt like me much either, mainly because I figured out her scheme and told him what I think is going on and he denys it or changes the subject immedietly. So she doesnt like him hanging around with me anymore, so now we dont talk as much anymore because of her. By no means am I jealous, it’s just that I have been best friends with him since 6th grade and I want whats best for him, because we have been there for eachother through a lot of hard stuf and girl problems together, so it hurts me when hes hurting (no h0mo, but were practically brothers) but what else is there left to do or to say if he is not listening? and neither of them have a lot of friends anymore because of how wrapped up in themselves they are. so EVERYONE hates this girl but him. So they just think im jealous that they hang out more then we do, but im not at all, im happy for him that hes in a good relationship. and when my girlfriend and I of 1 year broke up he pretended like nothing happened, and when she broke up with him, he calls me at 2 in the AM crying hystericaly.