My best friend always has my back. Sometimes they treat me not so friendly but I love her and her boyfriend! They have been fighting a lot lately and I don't know what to say to them because like I said I'm bestfriends with both of them.. Help me!

Say nothing. Just go with the flow and remain open. You can't control what they do only yourself.

Califdadof3 is right, you need to stay out of it or it could cause you to lose both of them as friends.