Should I say something or let it happen?

So me and my gf have mutual friends. Her best friend since middle school is my best friends GF. They have fought a lot and she told me she was plannin on breakin up with him if things didnt get better. Well I was with my gf when his girl was texting mine and there was something about do you want to see a picture of him. so i asked a pic of who of course. and she told me not to worry bout it. so i asked if it was matt the girls bf. and she didnt say anything. then anytime she got a text she would put the phone close up to her face to where i couldnt see it. which doesnt happen. so i asked more and more about it. and all i got was that it wasnt matt. so of course me being concerned for my friend i went through her phone to see what it was about. and it wasnt about matt it was about another guy and she was sayin that she wanted him so badly and all this other stuff. now i dont know who this guy is and i have never heard anything about him. My girlfriend told me that stacie the other girl was not gonna leave matt for another guy but from what i see its a little different. my gf told me that its not my place to say something to him about it. but thats my best friend and hes looked out for me and i know he'd want the same i think my gf is just sayin that so her friend doesnt get in trouble. so my question is to i tell him about what i know of his relationship that he might not know or do i wait for them to break up.