Well, the main problem here is that she's cheating of her boyfriend.
While a coming out is a very personal thing, cheating is just wrong (no matter the reasons).
My advice? Tell her to clear up things with her boyfriend at least (she can "just" broke up with him without coming out if she doesn't like him no more).
If she's not willing to, tell here you'll do it, and that's just fair unless you want to be an "accomplice" of the cheating.