These suggestions are on the basis that the demise of your friendship is over a disagreement of some sort, not for when you find out that your friend has been screwing your man behind your back!
Be the better person. I don’t mean that you should follow her around emulating her and exceeding her behaviours but if she fights dirty, it doesn’t mean that you should. Any ‘friend’ that wants to fight dirty with you as two grown adults probably isn’t worth your time anyway.
Don’t bad mouth to all and sundry. The demise of friendship can often feel like break-up of a relationship, even a marriage, so tempting as it is to tell every person what a bitch you think she is, don’t. People love gossip and bitchiness but when all is said and done, they still end up thinking that you’re childish or just plain bitchy anyway.
Keep the secrets to yourself. You may well make friends again one day or still be in touch with mutual friends and you will not be thanked for revealing her confidences to everyone. I doubt you’d like it if the tables were turned on you! Remember, what goes on tour, stays on tour, so there is no need to tell her boyfriend about the time when she snogged some random bloke in the first few months of their relationship.
Try to keep the side gathering to a minimum. It’s natural to make sure you have your camp and they have theirs, but be careful of making everyone feel uncomfortable about who to pledge their allegiance to.
No revenge please. Fatal attraction is for couples, Single White Female is for friendships. Neither of these characters are what you should be modelling yourself on. Nuff said.
Have a cooling off period. Much as you may be livid with her right now, it is possible that when the anger simmers and time does a bit of healing that you realise how much you want the friendship. Hence minimising the fall out from the disagreement until you’re sure you really want to end it will prevent you from closing the door after the horse has bolted.