This is a tough one, because dating a friend's ex is one of the most essential dating taboos. "Don't date your ex" is right up there with "don't break up over email." For good reason. Which is that people never really get over significant romantic relationships. Not really.
I don't mean that you can never be happy again after breaking up with someone. Of course you can. But it's largely a matter of compartmentalizing. You get a new and even prettier girlfriend, or hang out with your friends more, or get into jiu jitsu or knitting. You get a new life so you don't spend all your time sitting around and crying about your old one like the baby you are. Sure, people will talk about their old relationships and say that they're "over it" or that it "wasn't meant to be," or pepper you with other related nonsense phrases, but what they mean is they're not thinking about it right now. All of those old wounds stick around, just waiting to be re-opened.
You're always going to feel weird when you see your ex, at least a little bit. (If you're still friends with your ex, it wasn't that serious, or it's still serious.) And, if you date your friends' ex, you're telling your friend that your romantic feelings are more important than their happiness. Because they're going to see you holding hands with their ex, and remember how nice that felt, and if you think they won't envision you having sex together, you're being naive. Of course, that's going to hurt. Inherently, it's a selfish thing. You're saying, "screw you and your silly emotions, I've got to get laid." Even starting a discussion about this — even coming to your friend and bringing up the subject — will seem like kind of a dick move.
Does this mean you should never, ever date a friend's ex? Well, actually, no. It means that it's a bad idea, but that's not the same as "never do it." True chemistry between people is incredibly rare. It's possible that you've got an uncommon romance on your hands. Maybe this is your future wife, or muse, or whatever it is that you're looking for. If this is the case, you'd be missing out on years of potential passion if you passed up on this girl for the sake of sparing your friends' feelings. Like I said, this is a tough one. You've got a hell of a decision to make.
Accordingly, you should treat this like any other important decision, which is to say that you should get as much information as possible. First, be real with yourself. Is this girl really special to you? Is there actually an uncommon reaction between you — some sort of deep compatibility that's worth alienating your friend for? Or is she just an attractive person who finds you attractive, too? (Those two things are so, so easily confused.) Approach these questions with the maximum possible skepticism about yourself. If you feel fluttery every time you talk to a pretty girl, keep that in mind. If you're currently lonely and you really need to get laid, consider that maybe you're just desperate. And dwell on the fact that some of your excitement might just come from the taboo nature of this potential relationship, because, like everyone else, you want what's off-limits.
If I were a betting man, I'd bet that your crush on this girl is just like any other crush. It's a fun illusion, which, if pursued, might reveal a great relationship, or might not. Odds are, that's what this is, in which case you should probably just sigh, move on, and hit up your online dating site of choice, where you can find lots of other crush-worthy women.
I found out about this the hard way, in a similar situation. Tom, one of my childhood friends, was always kind of bummed out, until he met Josie, a fast-talking, high-energy woman who brought him out of his shell. They had a sparkling relationship — they were one of those couples that just radiated warm, gooey, nauseating passion — and I was super envious of it. Also, obviously, Josie was an outrageously foxy person: one of those girls where it feels like your eyeballs are being manipulated. There was only one weird thing about her, which is that I would occasionally catch her looking at me a little funny. I suspected that she had a low-key crush on me.
About a year after she dumped Tom abruptly, leaving him a sniveling wreck, I discovered that I was correct. We ran into each other at a party. Flirting with each other was easy, taking each other's clothes off was easy, and it felt like fate — like absolute magic — and it kept feeling like absolute magic for about a month, at which point I discovered that she was kind of boring, or at least that we bored each other. In the end, I got about five dates out of it, in exchange for an irreparably ruined friendship. Tom found out, and he never forgave me.
Now, if you're sure that this is not you, that this is Real Love you're looking at, then what you have to do is tell your friend what's up. Tell your friend about your intentions, and ask him if there's any way you can make the process easier for him. This conversation will not go well. Because, again, you're declaring to an old friend that you're prepared to disregard his feelings. That's the truth of the situation. There's no two ways about it. It's going to hurt even if you say all the nice things you should say — that you're still going to be his friend, that you're going to try not to Instagram this girl obsessively, etc.
Ultimately, dating a friend's ex is inadvisable. You know that. But sometimes, you have to do inadvisable things for love.