Is it ever OK to hook up with an ex’s best friend? What are the rules?

This probably depends on your relationship with your ex and how things ended between you. If your ex is now in another relationship, hooking up with her best friend is unlikely to give her cause for concern (unless you treated her so badly that she will be concerned for her friend, whereby she is likely to try and do everything to talk her out of dating you.)

It is worth being aware that your ex’s best friend will know everything that she needs to know about you (and this will include the bad bits because it will be your ex’s duty to pass this information on.) As such, it is a seriously bad idea to try to lie your way out of any situation – be honest and open and find a good explanation as to why you might have behaved in a certain way in the past. (If in doubt, blame yourself – not your ex.)

Lastly, you must never bitch about your ex (however tempting it may be.) There is an ‘unwritten rule’ between women and this is that you never betray a mate for a bloke. So if you say bad things about your ex, rest assured it will be repeated to her (and then she might just tell your new girlfriend one or two secrets about you that you were hoping she’d keep to herself.)

the short answer- it depends. If you don't talk to your ex anymore, then that's for her and her friend to work out. If you're friends with your ex, then it will ruin everything if she finds out.

Also, as DizzyLizzy said- NEVER mention your ex while around the friend. Ever.