Well, sleeping with someone who is already in a relationship is generally considered wrong, but IMO, given that you aren't forcing anyone to cheat, cheating on anyone, or friends with the person being cheated on, it's not really your concern, ethically speaking. But being that she's your coworker, there's a big chance of this hurting you. There's also the risk of a false rape accusation if he ever finds out and she takes the easy way out of the consequences of her actions. Finally, from what you've described, it sounds like she's using you for attention, and not genuinely interested.
Anyway. If you google around "boyfriend destroying tactics" you'll find some interesting info; to summarise, you wait for her to talk about him (or guide the conversation there yourself) and any time she complains about his behaviour, you reframe it as "nice guy" behaviour so that it sounds like you're defending him, when really you're making him sound needy, low-value and boring. If she says he texts too much, you say "you mean the world to him, so of course he wants to talk to you all the time". If he doesn't text often enough, you say "he's just worried that you'll realise he's a bit clingy and insecure, he doesn't mean to make you feel bad". The idea is to take all the mystery out of him and make him seem beta, while seeming as though you're defending him.
Try to be subtle, but I used similar tactics quite ham-handedly once and still succeeded, although mostly because the girl already had doubts - it's far easier to play off existing doubts than it is to seed new ones. The other thing you want to focus on is finding out what's missing from their relationship and offering her that. If he's too needy then you should be aloof. If he's too aloof, you should connect to her emotionally (without being an emotional tampon). You need to get her talking about him, but it's best if you don't directly ask because you don't want her to think you're trying to damage their relationship (even though you are).
The best thing to do would be to invite her to a bar after work. Casually say "Come with me to bar X after work. Just one drink. [smile] I won't take no for an answer [wink]". Get her laughing, do very light kino (google "escalation ladder"), then guide the conversation towards the boyfriend and try to use the above tactics. Once she's had a couple of drinks - just to loosen her inhibitions, I'm not telling you to date rape her - invite her back to your place for some innocent non-sexual activity. The best is if you have a pet, because bitches love pets. You can tell her "Rover is just dying to meet you". If she agrees to go back with you, then continue escalating. If she doesn't eject at any point, then you should get laid, but it's a big if; to be honest, I don't fancy your chances, or the risks involved (but I would still do it because I'm irresponsible).
