OK, no moral talk, just the "how to"

First of all, keep the interaction private, no flirting or kino in front of common friends, no invitations to gatherings where friends/acquaintances may see the two of you. Be discreet, don't brag about seducing her to your friends, for the sake of both parties...
Steer the conversation away from her relationship, as a matter of fact, don't let her bring him into the conversation at all. Don't let her rationalize what's happening, as it may be the end of it all when she realizes she's about to cheat on her boyfriend. Be the opposite of what her boyfriend is like, if he's into football and beer, you take her to see a foreign film...
Be decisive, it's now or never, don't wait for the right moment, you have nothing to lose even if she says NO.
Don't let her guilt trip you or herself, chalk it up to "The incredible attraction we have going", she'll feel like less of a "slut"...
Don't leave traces, like e-mails, Facebook comments and long horny text messages on her cell...
Hope it helps... Have fun!