@JWChibi Your way of thinking is extremely egoistic in my point of view. If two people like each other, why should both have to be unhappy because of a third party? The thing about abusive exes or whatever is something you made up in your mind and that has nothing to do with anything. And just because a break up happened one sidely and someone suffered, that doesn't make the other person a demon, sinner or abusive person in any way. Sometimes one side just falls out of love and it's not anyone's fault.
If the two people that are in love are single, free and haven't done anything wrong (there's been no such thing as deception, cheating on anyone or anything), there's absolutely no reason, in my point of view, that should prevent them from being together. Friendships should go both ways, why is one person's feelings more important than the feelings of love of not only one, but two other people? There should be balance in a friendship. If someone's feelings above the other's, then it's not healthy anymore. Or maybe it was never friendship to start with. Shouldn't the supposed friend with unrequired feelings wish for his/her ex-lover and friend happiness too? Everyone is the protagonist of their own lives. If your feelings are unrequired, them it's not the ex, the friend or anyone else's fault. That person just wasn't meant to be yours. One should then learn to move on with their lives, you can't force love. And just because it didn't work out with you, it doesn't mean that in won't work with someone else. It's okay to become bitter for a while because it's in the human nature. And it's also normal to distance yourself while you get over it and heal. But if you turn your back permanently because you couldn't deal with rejection, then it was never real friendship to begin with.
Besides, in this case, there's even the time factor. It's not like the relationship just ended, or that the ex broke up to go date the friend soon after. There's a 3 year gap between relationships in this.
What's more, in a situation which you don't have feelings for your ex anymore and still wouldn't want him or her dating a friend of yours, that is all about possession and a hurt ego. And that feeling is so ugly, not to say, immature.
In case which the "ex" is abusive, then the friend shouldn't want to see them together not purely because of the fact the guy's her ex, but because she cares about the friend and wants someone whos better for her as a whole (meaning the fact the guy's an ex is irrelevant).