Embrace your inner plumber.

The moment one of your friends mentions bf, interrupt her and explain he cheated on you and you need to be protected from this man as long as he doesn't change. A real friend will support you.

Prepare a simple summary of the situation and your plan (bf cheated and was having angry outbursts, until he changes his behaviour, I will avoid any contact, so can we please change the subject before it takes me back to the horrible situation I was in?)
Next time a friend mentions bf contacting her, ask her support and to protect you from breach of NC.