Whether or not you believe your situation is an exception, you should always talk to your friend before making any crucial decisions. Unless you value your relationship with a guy more than your friendship, respect that your friend may not be thrilled you want to start dating her ex. On the other hand, it may matter to your friend or even yourself, so tread carefully if that's the path you're choosing to take. Better yet, if she's in another relationship and is seriously in love, it's doubtful she'll care too much if you want to date her ex. If this is the case, and your friend is still concerned, it's best to stay away from the ex. Her hesitation is for a good reason.
Dating Your Friend's Ex
My Ex is Dating My Friend! How to Handle it and How to Keep From Going Insane | HuffPost Life
It all comes down to how your friend feels about it. You were a good friend and kept your feelings to yourself for long enough. It was a mutual breakup with no hard feelings. This is the ideal situation. Go ahead and ask him out.
When Is It OK To Date My Friend's Ex?
There are a few different things that happened during the time I was newly separated that caused me to classify myself as temporarily psychotic, one of those being when I figured out "my ex is dating my friend! I seriously felt like I was going to go insane. Here was a woman who I thought was my good girlfriend. We had gotten together a few times and I had told her things -- personal things about my ex and our relationship, why I was getting divorced, etc.
In the passenger seat was my high school boyfriend. We had become infatuated with one another senior year, and we were now facing the inevitable relational shift: Our chosen colleges were three hours apart and neither one of us had a car or money. I was devastated, but a few weeks had passed and we were still in semi-regular contact. I would sign onto iChat everyday, and we spoke as if nothing really had changed.