This problem needs to be settled NOW!!!?

Okay, my former best friend(lets call her 'K') dated my cousin('A') all of 8th grade year. He was in 9th grade(keep in mind he's like the older brother I've always wanted & I love him like he's my brother) They contemplated getting married & me, being the stupid kid I was, told her it was an amazing idea & encouraged her. They broke up almost a year in2 the relationship & he's never wanted a relationship with her since. Well she decides to drag her anger out until now & it still flows. She's always talking down to him and rants about him to me, knowing that I think of him as a brother. She knows I hate it. Also, all three of her relationships she's had have lasted @ leas 8 months & involved engagement & she lives with her current 'fianc�'. 2 days ago my current best friend(Lauren) posted a song lyric & my cousin commented with the lyric that comes right after that. K decides to type " I'm starting to wonder why you like to stalk you ex's"( A dated Lauren for a month & they're still friends). Lauren says "not on my status" so they'll take it some where else. K proceeds to call A some names & A is pretty angry by now so he says 'Hoe please, you better bow down in both knees' since K NEVER finishes what she starts she drags her bf into it & he calls A a spick. A leaves a comment & idk what the **** he's talking about & he never comments back. I'm not condoning what A said but he's been cool the whole time K has been riding his ***. It seems like she never got over their break up. Some days she says **** like 'he was my first love, I'll always love him' & other days she's cussing him oy behind his back. & she never has to hear from him. She lives in Texas & he moved to Kanas (:'( ). She still has him as a friend on FB & I tell her all she has to do is delete & block him & she always comes up w a lame excuse 2 keep him. 4 years is long enough. Every1(but me, I'm 17) is in their freshman/sophomore year of college & it's time to grow up. Do I intervene? What do I do?