I don't care how long you've been friends, how deep your friendship is, or the outside circumstance, if you want to maintain your friendship with someone, you absolutely CANNOT pursue your friend's ex. Sorry, it just had to be said. Unless there's some sort of friendship consensus or said agreement that her ex-boyfriend is totally up for grabs, I would not be barking up that tree.
No matter the breakup circumstances, more likely than not, there's probably history, hurt, and pain associated with your ex. Some things are hidden and others you've gotten over, but if your friend goes after your ex, it's almost like it nullifies anything hurt, rejection, or issues you went through during your break-up.
Plus, it calls into question the loyalty of your friend. How loyal is this friend to me if she was willing to go behind my back to talk to my ex? Is she really that loyal if she's been deceiving me or acting like pursuing my ex won't affect me at all? Come on ladies. This is like the number one of all rules when it comes to friendships. If you care about your friend, don't mess around when it comes to her past relationships. Know that there's plenty of fish in the sea and your prince charming is awaiting you.
You always have a choice.
Don't date flippantly - have wisdom enough to know that your actions can affect others. How much is pursuing that boy worth it to you? Is it worth running a friendship under the bus? The least you could do is be mindful of how your choices may affect your friend.
It is honoring to your friend if you choose to stand by her instead of taking aim at something so close to her heart. This means that talking to her ex, flirting with her ex, catering to her ex, or housing her ex are all off limits.
Of course, there is the rare circumstance in which case your ex and your best friend end up living happily ever after. And if that's the case, I hope you offer your full support on their relationship.
After all, what's that saying? Sisters before misters. Let's start honoring our friendships a little bit more because loyalty is necesary for high quality freindships.