It doesn't matter how long you dated your ex, who broke up with who, or if you're still sometimes hooking up with him, it still feels like a slap in the face to find out your friend is dating him. I mean like Gretchen Weiners said, ex-boyfriends are just off-limits to friends, that's just like, the rules of feminism. But the reality is that it happens, and sometimes it happens to you.
Obviously it's not fun to find out that you and your friend have called the same dude your boyfriend at one time or another. If she's not a close friend, your first instinct is probably to shut her out for being a backstabbing bitch. Either that, or you suddenly want to be best friends with her, so you can get all the dirt. If she's a close friend who is not a Shady McShadester, chances are you're way over your ex and the only reason she thought it was okay was because she knows you're totally over him. Still, it's like, when you donate your high school Guess bomber jacket you definitely will never wear again to charity, you don't expect anyone you know to actually buy it and wear it in front of you. Same concept—even if your friend didn't mean to fall for your ex, you can't help but wish he just moved to Europe after you guys broke up instead.
Dating your friend's ex might be up there with Watergate on a scandalous level, but the reality is you and your friends are probably both attractive, chill betches and it's not unlikely a bro would fall for both of you. Plus, the fact that your friend likes your ex means that she thinks you have good taste. All that being said, you don't have to be cool with it.
If you're annoyed by the fact that she's now parking your second-hand car in her garage, literally everybody you know will understand. While your friends might not actually take sides on the issue, they'll def sympathize with you (then afterwards you should dump them all for not taking your side, because you're totally in the right). Instead of acting cool with it when you're not, you can make a choice to call it out as being fucked up. You're not obligated to hide your feelings, because they didn't. If you think about it, your friend made a choice too, and it wasn't you.
Watching your friend get the same compliments on that bomber jacket that you used to get is going to make you wonder if you should have just held onto it. But then you remember that there's no fucking way you'd wear that unless you were going to a 2000's theme party, so it would just sit in your closet collecting dust, anyway. (This is an extended metaphor for your ex, BTW). If you've moved on and want to keep both your ex and your friend in your network, just remember that all it it means—aside from you being a fucking saint—is your ex thinks you have good taste in friends, and your friend thinks you have good taste in exes. I mean, Cady does end up dating Aaron at the end of Mean Girls, and we all rooted for her anyway.