I can answer this from my perspective of a best man that tried to make something similar happen.

It's pretty likely this particular best man was just a douche nozzle, but in our case it was a last ditch effort to break them up that was unsuccessful. She was (and still presumably, she doesn't allow him to see friends without her supervision) horribly abusive both physically and mentally. She constantly screamed at him and slapped him across the face for any and all slights, be they major or minor, real or imaginary. That behavior extended to us as well. I hurt my back when helping them move when she was shoving me down the stairs while I was holding a dryer because I wasn't moving fast enough for her liking. She just sat and screamed at us to move faster and when the shoving down the stairs happened, I stumbled and caught myself before dropping the dryer or falling, but at the cost of my lower back muscles... I'm not exactly proud but I had to be restrained from quite forcefully correcting this behavior. She was a slave driver that day.

Another example, and again, this was a normal day, was when a few of us were with him at his place to meet her for the first time. I said something smartass to the friend, he laughed and called me a cunt. She slapped him so hard his glasses went flying across the room and she screamed "IVE TOLD YOU ABOUT THAT WORD DUMBASS" and then turns to me and the other friends "SAME GOES FOR YOU!"

This cunt has zero redeeming qualities, but my friend had never had another girlfriend ever. We tried to tell him how terrible she was in every way, but he wasn't listening. So trying to get him to talk to other girls at his bachelor party was a last ditch effort and we felt scummy about it, but there was no other way to save him. It didn't work.