The Brahma Kumaris "win" because they have a gorgeous (albeit cold) mansion house and gardens in the country and offer New Agey courses for free. If you can deal with ignoring the very minor guilt hints about making voluntary donations, you might even get away with a few free meals too. part of the appeal is that they can make you an instant guru in a kind of Forest Gump way.
The New Agey courses are not what the Brahma Kumaris are all about. The real teachings are here. This is allegedly how "God" - the one and only one - speaks to humanity ... and exclusively to the Brahma Kumaris.
The New Age courses are the warm bath to entice individuals in. The real meat starts to take place in the mediations at the end of all the waffle. During the waffle, the BKs will appear to be interested, enthusiastic and listening to your partners opinions etc. In truth, they are just sitting there meditating 'at' her ... zapping her with their psychic vibes ... doing what they call "dhristi", a kind of staring in which they imagine linking up to their god spirit and then hook up her, or they actually experience being used as a channel or medium for that/those spirits.
These are not all the words the BKs uses but I am trying best to explain it in terms you might understand. The meditation is a prelude to mediumship ... many people do not get beyond the simple sitting peacefully listen or meditating phase.
The real BK course used to be called "The 7 Days Course". They do not teach this straight off any more because it is too whacky. So they will entertain her, befriend her, "take her pulse" until she is "ready", i.e. gullible or defenses have dropped enough, in order to go to the next level or full indoctrination. The 7 Days Course is now also often split over 2 courses and so the initiation into a relationship with this spirit entity may take a period long time; weeks or months.
I think the best thing you can do is go straight to the teachings. Print off the posters, here or here, print off the teachings manuals or books, print off a few Murlis and leave them around so she can see them. Read them yourself. Ask her if she agrees with them. It is probably a better strategy than fighting it. Note though, the BKs are likely start to give her all sorts of "yuktis" to not answering straight or questions. They will likely not be not answering straight or her questions at this point. It will all be very vague and appealing.
Alternatively, do not make it obvious and just keep her otherwise entertained. Love her well and make sure the sex is good because is she gets sucked in, that will be all over.
If she asks what the BKWSU is all about, you need to make it clear that the BKWSU is not about waffly stuff like, "our original state of being" , "the light", "the one", "the innate goodness of all human beings" (that last one even goes against their own teachings). It is about hooking people up to a spook, a disincarnate spirit who claims it is the god of all religion ... called "BapDada" ... who possess an old lady in India and speaks through her.Of course, perhaps she might find that interesting/appealing/intriguing at which point you might have a problem ...
Therefore, in essence, if she is serious about them, educate yourself a little bit about them. Buy the Dr John Walliss book and read it. Read what is here.
Is she the sort that if you push too hard she might react and go in the other direction to them?