Why is my friend so p*ssy whipped?

my friend and his gf have been dating for about a year now, she has played him countless number of times, he caught her with another guy at her house spending then night and she didnt tell him, she even todl one of her friends it doesnt matter how i treat him i can do what ever i want when ever want, cause i know that he will always come crawling back, he has even wanted to fight ppl over this girl, and when i try telling him he is an idiot and hes going to end up going to jail over a b*tch, he doesnt belive me or he goes ya ik and laughs(when ik deep down he does not take what i said to heart), once we even yelled and screemed at eachother when we were at a resturant, he has done everything for this girl and she doesnt even show one bit of concideration, he always gets latched on to girls like he obsessive over them all the time but most of them get creeped out by it and dump him, but not this one, so what i am trying to ask is why does he act like that and is there anything i can do or say to him to make him realize that she is not the only one out there, ive tried everything, it seems like everything i say to him he doesnt take in, its more like i clean up his bed and then he messes in it again