1. Don't Blame Yourself
We're supposed to be there and protect our girls from dating these jerks. I know. But sometimes, no matter what you say or what you do, one falls through the cracks. Or he's a douchebag in disguise. So just know that sometimes you can't do anymore than you have already done and you can't blame yourself for her relationship. After all, it is her relationship and you don't have control over EVERYTHING.
2. Practice Self Restraint
Chances are you're going to have to hang out with this jerk at least once. And he is probably going to offend you in some way, whether it is by continually making rape jokes while everyone in the room gets more and more uncomfortable, being blatantly racist against you in particular every time he sees you, or making fun of your obvious feminism, you're going to have to hold your tongue because this is someone your friend cares about. So do some conditioning. Or buy a journal. Or get a blog. Throw shade in a less obvious way than saying directly to his face "you're an acne scarred troll who knows nothing about anything and I kind of hate you so take yourself and the rest of your brony gang back to the place you came from and never come in my house again" is probably not the best way to keep your friend.
3. Never Be Fake
I only said you had to be polite. I never said you have to completely hide the fact that you do in fact, hate the human in question. Use your intelligence to undermine everything they say or do. This does in fact seem petty (probably because it is) but also just wait to your friend goes away for a bit and explain in great detail just how much a flaming asshole you think they are and that you only tolerate them because your friend is "in love" with them. Continually point out more attractive, generally better, potential dates to your friend in the presence of their S.O. ask them if they think your target would be a better match for your friend.
4. Don't Say "I Told You So"
Chances are you're going to tell your friend how much you hate their boyfriend. I know I did. Often. I also insulted his intelligence quite often because I believe that the only worse thing than a douchebag of a boyfriend is a stupid douchebag of a boyfriend. But when they inevitably start treating your girl bad and become a totalitarian dictator rather than a boyfriend, support your friend. Support her and tell her she is a free thinking woman who can do whatever the hell she wants and doesn't need any man to control the way she breathes, walks, and talks. This will make her significant other hate you more than he already does, but who cares? You're not dating him. However, don't rub it in her face that things aren't working out just like you said they wouldn't. She already knows. She doesn't need that from you.
5. Give Her Strength
Sometimes the hardest part about breaking things off with a douchebag is that they usually stick around for a while. Like 2 years. I dated a douchebag for an entire year. Why? Why do we do that? Because we feel that no one will love or care for us like they do, even if they are rough about it. So when your friend says she is thinking about breaking with her ball-and-chain, give her the strength she needs to actually do it. Let her know that you will still love her, and that if love comes at the price of disrespect it isn't worth it. Be the wind beneath her wings. Let her soar.
6. Don't Blame Her
Wolves hide in sheep's clothing. Like I said earlier, maybe he was sneaky about revealing his douchiness until the very end. You can't blame her for making a bad relationship decision. She's only human and can't see through people's personalities. The only one who should be getting blamed for how horrible the douche was is the douche in question.
Once you live out the relationship through to the end at your friends side, party. It's over. You did it. If she dates another asshole ever again smack her. But for now, take her out for some celebratory coffee and lunch. She deserves it after dealing with that jerk for so long.
Alexis Olmstead is a 20something diva in the stage of life where everything is either awesome or awful. She believes that people shouldn't date until they're over 25 when their prefrontal cortex develops but hey. That doesn't stop her! You can find her in her room playing Sims and scribbling down blog ideas that she'll get to sometime next year.