One thing you need to consider, if you're thinking of trying this approach: do you really think that your new spouse or GF someday will be cool that you're "best friends" with your ex-wife?
Be civil, be courteous, be friend-LY. If this doesn't work out, you'll need to co-parent, and those will all serve you well. But if your wayward spouse is under the illusion that you're going to still be BFF with them if they continue to cheat and run away from your marriage, I think you'd be wise to set them straight, in a calm, firm, loving manner.
