Few of my friends would be married if ex’s or crushes were off limits. And yet, we’ve married ex’s, married crushes, and stayed friends. It really isn’t a big deal if you are emotionally older than sixteen and everyone involved are decent human beings.
Then again, there is a corollary of trusting your bro “with your life, but not your money or your wife.”
Really it depends on the nature of relationship. Just because you have a “crush” on some girls does not make her off limits to the rest of the world. It is poor form to cockblock your friend if he makes his intentions known. However, after reasonable attempts are made at initiating a romantic encounter, she becomes fair game. You don’t get to place the pussy on layaway in the off chance she will like you years from now.
If it’s just random hookups and casual dating, anything is fair game. In high school and college, dating some girl or guy who had just been dating your friend was pretty common.
Dating your friend’s ex-wife or ex-serious girlfriend is another matter entirely. It’s not out of the question, however there are certain protocols that must be followed. Like if your best bro has spent the last month sulking around his apartment surrounded by old pizza boxes and whisky bottles, it might be a bit too soon to you to ask for his exes number.
A good rule of thumb is you can’t start seeing the ex until your friend has started dating again. The reason for this is obvious. The friend is trying to move on, which is a little hard to do if you keep bringing his ex around to all your social functions.