When I was in the University, I had this "friend" , note the quotation marks, that had a boyfriend who did everything to make her happy but she treated him like an outcast, eventually they broke up but I got to know from the dude. Anyway, I saw it coming.
The breakup was during the ASUU strike so when we resumed for our project defence, the two man squad became a one man squad. Since the dude didn't offend me, we were still keeping in touch.
One fateful day, he called me that he was in front of my college building, so I went out to meet him and we sat in his car gisting for a long time and I can be dramatic with my gesticulations. Somehow she got wind I was in her ex boyfi's car and I think she was watching from a distance. She later went on to tell a mutual friend to herself, ex boyfi and me that since I knew she wasn't dating him anymore, I should keep malice with him. I just told her to grow up.
While growing up, if one person picked on my circle of friends in high school, the enemy automatically became my way too but later events proved me wrong as the two people involved would reconcile without my knowledge, making me look like a fool.
The decision to make the enemy of your friend your enemy or friend is one that you should be cautious about. If you can, always listen to both sides of the story so as not to end up fighting a battle you know nothing about.
Besides, adult mend fences while kids tear fences apart.